The government's various incentive policies (such as subsidies, free vehicle licenses, etc.) play an essential role in the purchase of private electric vehicles by consumers, but the government cannot implement strong incentive policies forever. If the incentive policies are gradually weakened or even wholly withdrawn, will the diffusion of private electric vehicles continue? What kind of mechanisms for incentive-policy-backslide should the government use in order to make less impact on the diffusion of private electric vehicles? After a rapid diffusion of private electric vehicles, the government and industry have begun to think about these issues. Based on patent data, this study first analyzed the technology maturity of electric vehicles by using life cycle theory of technological development. Then this study clarified the causal links between important factors which promoted the diffusion of private electric vehicles, such as government incentive policies, charging pile ratios, and technology learning effects. The feedback loops formed by these important factors were also analyzed. On this basis, this study built a system dynamics model for the diffusion of private electric vehicles. The innovation of this model is to introduce technology maturity into the system dynamics model. Shanghai is the city with the largest number of electric vehicles in the world, which is mainly due to the government's strong incentive policies (e.g. free vehicle licenses). This study used the historical data of Shanghai to verify the system dynamics model, and then used this model to simulate the diffusion scenarios of private electric vehicles in Shanghai under different incentive-policy-backslide mechanisms. The research results can provide references for the formulation of incentive-policy-backslide mechanisms for electric vehicles, and also enhance the prospects of alternative fuel vehicle manufacturers for future markets.