Based on the analysis of 2760 employees from 23 public organizations in Beijing, this paper employs the mixed research to explore the effect mechanism of social network on public organizational knowledge sharing by using structural equation model and qualitative comparative analysis (QCA). Results showed that: Firstly, there is a positive correlation between social network and knowledge sharing willingness. Secondly, positive emotion and relationship quality play a significant mediating role between social network and knowledge sharing willingness. Thirdly, social network expertise heterogeneity * social network density * positive emotion，social network general heterogeneity * social network expertise heterogeneity * positive emotion or social network density* relationship quality * positive emotion is a necessary condition for knowledge sharing willingness. For a particular public organization to promote its knowledge sharing, it is necessary to create certain conditions in conjunction with its own endowment. The research conclusion reveals the mechanism of social network on public organizational knowledge sharing, expands the existing results of the public knowledge sharing behavior, and has a strong role in enriching public social network research and stimulating public knowledge sharing in a dynamic competitive environment. It also provides an important empirical basis for building a governance community where everyone has responsible, everyone is responsible, and everyone enjoys.