Based on a thought experiment that distinguishes between internal and external risks in financial transactions, this paper constructs an analytical framework to examine how external investors’risk perception affects the equilibrium outcomes in the securities market. A controllable and replaceable securities trading experiment is conducted to empirically test the theoretical hypothesis. The results indicate that investors are more sensitive to external risks. When external risks reach a critical threshold, changes in external investors’coping strategies can trigger abrupt fluctuations in market returns and trading volumes, demonstrating both price effects and quantity effects of external risks. The findings suggest that the fundamental cause of the “sharp rises and falls” in China’s securities market may lie in the moral hazards and opportunistic behaviors of corporate controllers.