[Purpose/significance]Social emotions in emergencies could affect network public opinion, which was an important reason for abnormal changes in network public opinion. The research on the evolution mechanism of netizens' social emotions in sudden network public opinion may provide a new idea for the governance of network public opinion. [Method/ process]Based on the principles of social psychology, four classifications of positive social mood, neutral social mood, anti-social mood and alienated social mood were proposed. On this basis, the formation mechanism of social mood in network public opinion of emergencies was constructed. [Result/conclusion] By analyzing the psychological characteristics of netizens in different stages, this paper put forward a series of strategies to control network public opinion by using social mood. During the period of social mood brewing, the government needed to strengthen the supervision of social mood perception; during the period of social mood awakening, the government needed to differentiate the interests of netizens and give reasonable channels to release them; during the period of social mood outbreak, the government needed to timely communicate with netizens and provide emotional comfort.