Social networks among members are necessary conditions for high achievement, but few studies are done about the factors which determine the CSCL social network. In this study, we identified and explored three factors: group structure, member?s physical location distribution, and member?s social position, which are considered having great influence on online social network. Our research relies on the analysis of a real, long-term, complex collaborative learning case. The result shows us the three factors do have a great impact on the CSCL social network. The proof of this assertion is the major contribution of this paper.