The social Q&A community, as one of the most popular platforms of knowledge sharing, now faces user churn and slower growth in new active user numbers. To some extent, the main problem concealing in the decrease of growth is the lack of knowledge contribution from the users, which is the most attractive function of the social Q&A community. Thus the key point of tackling this problem is to explore which factors influence users' knowledge sharing behavior. Based on social capital theory and social exchange theory, this study constructs a regression model and collects empirical data to investigate the influencing factors of users' knowledge sharing behavior. It is found that the increment of users' centrality, harvest and users' trust toward the community lead to more and better shared‐knowledge. However, the increase in users' reputation has a significant negative impact on the quantity and quality of their behavior. The theoretical and practical implications derived from the findings are further discussed.