The thesis aims to explore the critical influential factors in changing a group’s collective decision and applies the in-depth interview method by interviewing six people with overall ten times group meetings of student activities. Based on the interview data, the thesis summarized in the following critical influential factors: providing new information, the timing of providing new information, the interpersonal relationship and expression ability of the proponent, the support by moderator, the agreement by the group members, and the discussion model, decision model, and execution model of the meeting. According to the conclusion and recommendations, the study provides some advice for future research to explore the critical influential factors as references.