In the contemporary society, the ability to work as part of a team and to coordinate the efforts of a team is becoming more important to the advancement of knowledge and the success of the employee in any job. The learning theory of constructivism indicates that, the success of collaborative learning depends on whether the interpersonal interaction among the group members is adequate. With the strong interactive function, the web provides the effective communication platform for distance learners to learn collaboratively. However, the positive benefits do not happen automatically in a virtual collaborative learning environment, which call for sound instructional design. This paper focuses on the interpersonal interaction in collaborative learning process. Based on the web-based collaborative learning (Web CL) platform which was designed and implemented by the School of Online Learning of Renmin University of China (CMR), we selected the discussion forum of the undergraduate web course "Dream of the Red Chamber" [Chinese characters omitted] as the case study and the students who chose the course and the instructor of the course as the respondents. We investigate the "interactive factors" which we assumed may influence the students' performance of interaction, then specifically evaluate the current situation of the "interactive factors" in the discussion forum and identify its weaknesses of interaction, finally put forward the enhancing strategies referring to the weaknesses. The study exploited the observation, and questionnaire survey method to acquire the quantitative and qualitative data and use content analysis, correlation and regression methods to uncover interesting results via data mining processes.