To motivate learners to engage in writing courses and help to improve their writing performance, a mobile-game-based collaborative prewriting approach was proposed in this study. A quasi-experiment was implemented by recruiting two classes of non-English major students from a university in northeastern China. Class 1 learned to write under the guidance of the mobile-game-based collaborative prewriting method, while Class 2 in the conventional teacher-centered approach. Their writing performance was measured to evaluate the effectiveness of the proposed approach. By analyzing the scores of students' writings in pre-test and post-test and the content of student discussions in prewriting activities, it was found that, the students in Class 1 who focused on content talk in group discussions improved more in organization of content, originality, fluency, and elaboration than those in Class 2 in post test. In addition, the findings showed that the incidence of different types of idea units in collaborative prewriting discussions had significant correlation with the ratings of different dimensions, respectively, in individual writings. Furthermore, the majority of volunteer students who finished the questionnaire endorsed the mobile-game-based collaborative prewriting approach.