Abstract: Against the backdrop of coordinated advancement of the new power system and the "dual carbon" goals, the growth of China's fuel cell electric vehicle (FCEV) stock still significantly lags behind planning targets, urgently calling for a systematic analysis of its diffusion mechanisms and policy effects. Therefore, this paper adopts the system dynamics (SD) method to construct an integrated simulation model incorporating key factors including technological progress, energy economic conditions, infrastructure development and policy support. The model systematically characterizes the evolutionary pathways and complex feedback structures of the FCEV market under multi-policy interactions. A variety of scenarios are set to simulate the dynamic impacts of different policy instruments and their combinations on FCEV stock, market competitiveness and carbon emissions. The analytical results indicate that, 1) although the FCEV market exhibits certain endogenous growth potential under the benchmark scenario, the internal positive feedback mechanism of the system has not been fully activated, making it difficult to achieve large-scale diffusion spontaneously. 2) Technological progress and R&D support provide fundamental support for long-term industrial expansion by reshaping the cost reduction trajectories. Hydrogen production electricity prices and energy efficiency exert significant amplifying effects on market scale through operating cost channels. Hydrogen refueling infrastructure exhibits phased characteristics evolving from "rigid constraint" to "network effect". 3) Multi-policy synergy can significantly amplify the effectiveness of individual policy instruments, which serves as a key condition to drive the transition of FCEVs from policy-driven diffusion to market self-reinforced diffusion. This study uncovers the internal mechanism of China's FCEV industry from the perspectives of system evolution and policy synergy, and provides quantitative evidence and decision-making references for optimizing policy portfolios and improving the efficiency of low-carbon transition in the transportation sector.