Users' contribution behavior plays an important role in the development of Q&A websites. This paper researches users' contribution behavior of the Q&A website from the perspective of MOA .Through the model construction and empirical analysis of users' contribution behavior, this paper investigates users' contribution behavior from motivation, opportunity and capacity. The results showed that: the motivation including altruism and reciprocity; opportunities the platform provided such as perceiving of ease of use, perceiving of economy, perceiving of image and atmosphere of website,and the ability including the user's own knowledge transformation and the professional knowledge, are related factors of users' contribution behavior. This paper analyzes users' contribution behavior from personal and organizational so as to provide ideas for the research and for the Q&A site's motivation mechanism.