Abstract: The effect of improvisation is closely related to the fit of the situation. Based on event system theory, and integrating the internal logic of "event-behavior-performance", this paper explores the impact of major emergencies on ambidextrous improvisational behavior, and elaborates its mechanism on entrepreneurial performance. Through an analysis of survey data from 268 entrepreneurs reveals that the more significant the characteristic attributes of major emergencies （novelty, disruption and criticality）, the more likely entrepreneurs are to engage in ambidextrous improvisational behaviors; In terms of mechanism, the novelty of major emergencies positively influences entrepreneurial performance through exploitative improvisation, criticality influences entrepreneurial performance through ambidextrous improvisation, while disruption only influences entrepreneurial performance through explorative improvisation.