The sustainable development of the new energy vehicle (NEV) industry is receiving increasing attention worldwide. However, as a “catch-up” country in the automobile industry, China has made remarkable achievements in NEV industry development. To explore this phenomenon, this paper develops an “innovation-demand-policy” (IDP) framework to investigate the driving forces of sustainable development of the NEV industry from the perspective of an innovation ecosystem. Based on a comprehensive data collection and processing of interviews, patents, industry reports, and policy documents, the findings showed that technological innovation, market demand, and government policy drive NEV industry development together, and policy can play an effective role of coordination only when it follows an innovation process and market demand selection mechanism. Specifically, technological grafting, potential market demand, and supply-side policy create a minimum viable ecosystem and the embryonic form of the NEV industry. Technological breakthroughs, public demand, and demand-side policy enhance the NEV industry’s ability to grow via a platform ecosystem. Additionally, total innovation, private demand, and environmental-side policy upgrade the NEV industry through expanding and reconfiguring the innovation ecosystem. This study also provides suggestions for policymakers and industrialists to promote sustainable development of the NEV industry in the future.