With the continuous development of Web 2.0 technology, social Q & A platform has become a new way for users to contribute, exchange and acquire knowledge. Based on the analysis of MOA（Motivation- Opportunity-Ability）and grounded theory, this paper constructs the influencing factors model of knowledge contribution behavior of the users of the Q & A platform. The results show that the three cores of MOA analysis can correspond to four main categories: motivation-perceived return and emotional satisfaction; opportunity-environment guidance；ability-self efficacy. These four main categories have a significant impact on user knowledge contribution behavior. On this basis, this study puts forward countermeasures and suggestions to promote the knowledge contribution of users.