The car purchase intention of noncar owners is closely related to the growth of car ownership and may be changed in the context of COVID-19. This paper aims to investigate the decision-making mechanism of the car purchase intention before COVID-19 and the change of car purchase intention after COVID-19. The contributions of influencing factors are derived from the gradient boosting decision tree model and the asymmetric effects of attitudinal factors are further analyzed based on the three-factor theory. The comprehensive importance hierarchies of the two dependent variables are constructed through the integrated analysis of impact range and impact asymmetry. The results show that people who were previously more willing to buy cars are more likely to increase their willingness to buy after COVID-19. The pre–COVID-19 car purchase intention is primarily determined by shared mobility-related attitudes, while attitudes toward private car use have a greater impact on the post–COVID-19 intention change. These two attitudes are mainly manifested as intention relievers and discouragers before COVID-19, but they are more likely transformed into intention strengtheners and encouragers after COVID-19. The availability of shared mobility has the maximum comprehensive importance to the post–COVID-19 intention change. Therefore, maintaining and promoting the ridership of shared mobility will be the most important prerequisite for alleviating the car purchase intention after COVID-19 pandemic.