On the basis of comprehensive analysis of existing buy site, it proposes a knowledge library structure of group buying websites, which profit model is based on membership, members' behavior. And providing support to the group buying site decision-making by using data mining technology to analyze the conduct of its members and to predicting further member's behavior. This study is not only extremely important to group buying sits' decision-making, but some common technology will also be researched in study of the project, e.g. the data mining technology of the members' behavior, etc. These common technologies will have some important meaning to the member based commerce.