Chinese citizens' participation in COVID-19 prevention and control has made great contributions to the successful fight against the pandemic. The factors that have impacted citizens' participation have rarely been reported based on both social-psychological and political environmental theories. This paper presented a study to explore the determinants of Chinese citizens' participation in COVID-19 prevention and control based on a combined model of the theory of planned behavior, the norm activation model, and political opportunity structure theory. A dataset involving 463 respondents from Harbin in northeast China was acquired and analyzed. The results showed that the comprehensive model explained 62.9% of the total variance in citizens' participation behavior. The openness to public participation not only significantly directly influenced citizens' participation but also indirectly affected participation behaviors through attitude and perceived behavioral control, both of which were important mediators and had the greatest overall impacts. The awareness of consequences and subjective norms were crucial antecedents to the activation of other influencing factors. Personal norms indirectly affected participation behavior through the mediation of attitude. The empirical results showed the comprehensiveness, effectiveness, and high explanatory power of the postulated model. The study also provides both new theoretical perspectives for explaining public participation and useful practical implications for future policy development in promoting citizens' participation in public health emergency management.