Abstract: In recent years, China's new energy storage has entered a large-scale development phase, during which energy storage companies focus on sustainable development goals and green development directions, and actively pursue green innovation strategies. Energy storage is a hot topic in the energy field, and from the field perspective, it is also a typical emerging field, and the emerging field is characterized by high volatility and instability of organizational roles and systems, for which innovative enterprises may face the dilemma of institutional vacuum. At the same time, power grid enterprises have been in a monopoly position in the energy field for a long time, and energy storage enterprises, as a new entrant to the industry, belong to a relatively vulnerable enterprises. As we all know, legitimacy is a necessary condition for the survival of enterprises, so it is of great practical significance to overcome the above difficulties and challenges to realize the legitimation of green innovation. In terms of theoretical background, the mainstream assumption of existing green innovation-related studies is that green innovation is a legitimate means for enterprises to comply with environmental regulations and satisfy the market's green demand, but this assumption is only applicable to explaining compliance green innovation, not strategic green innovation, which emphasizes differentiation and is ahead of the established institutional logic, and its legitimation process needs to be clarified urgently. Moreover, existing studies have rarely analyzed the legitimation process of green innovation in combination with a field perspective, and the legitimation process of green innovation has not been fully integrated with a field perspective. In addition, existing studies rarely analyze the legitimation process of green innovation from a field perspective and pay insufficient attention to strategic green innovation by disadvantaged firms in emerging fields. In summary, the core research question of this paper is: how and why is the legitimation process of strategic green innovation by disadvantaged firms in emerging fields possible? In order to answer this question, this paper focuses on the energy storage, an emerging industry, adopts a longitudinal single-case study method, and selects Global Mainstream Dynamic Energy Technology Ltd. as the object of study based on the theoretical sampling principle. The uniqueness of this case is that implementation of the terraced utilization of retired batteries belongs to the strategic green innovation of subversive significance, and the enterprise realizes the co-evolution with the institutional environment in the process of innovation through the perception of external opportunities, eventually dominating the industry norms. We obtained the required data through interviews with the executive team of the case firm, secondary data collection and on-site observation, and analysed the data using the coding method of the Grounded Theory, and then constructed a process model of the strategic green innovations legitimation by disadvantaged firms in the emergent field based on the results of the coding. In this paper, the strategic green innovation process of the case firms is divided into three stages: exploration, breakthrough and deepening, following the logic of “legitimacy challenge-legitimation process-legitimation result”, and introducing the perspective of organizational identity work to explain the internal mechanism of the legitimation process. The results of the study show that in the exploration phase of innovation, enterprise faces “cognitive pressure” and obtains legitimacy through “identity extending” and “innovation environment adapting”; In the innovation breakthrough phase, enterprise faces “cognitive pressure” and “normative pressure”, and gains legitimacy through “identity reinventing” and "innovation process designing”; In the innovation deepening phase, enterprise faces “cognitive pressure”, “normative pressure” and “regulatory pressure”, and mainly obtains legitimacy through "identity strengthening" and "innovation models diffusing". It can be observed that as the stages of innovation progress, the case firm becomes more dynamic and the level of legalization increases significantly. The findings of this paper include: (1) Disadvantaged firms in emerging arenas face the dual challenges of institutional vacuum and identity vulnerability, and the evolution of the arena's institutions in this context has led to different combinations of cognitive, normative, and regulatory pressures that need to be overcome for their strategic green innovations. (2) Firms’ organizational identity work and innovation strategy actions together constitute the legitimation process of strategic green innovation, which helps firms to achieve legitimacy in terms of professional identity, technological routes and ecosystems, respectively; within stages of the legitimation process, identity work underpins legitimation, sets the organizational strategic direction, and helps firms to escape from the identity vulnerability of new entrants. (3) Between stages, identity work creates identity spillover effects of "existing identity buffer" and “multi-identity claim transfer”, which is an important mechanism to promote the legitimation process of strategic green innovation across stages. This paper takes energy storage enterprises as an example of strategic emerging industry actors, theorizes the role of vulnerable enterprises in the emerging field, and provides theoretical support and practical guidance for their strategic green innovation legitimation characteristics and processes.