Collaborative writing is a pedagogical activity increasingly used in second language education. Despite the increased research on L2 students' participation in collaborative writing, scant attention has been paid to student engagement in project-based collaborative writing and the possible engagement changes. This article reports on a longitudinal case study investigating the changes in student engagement of a group of five Chinese EFL undergraduates in project-based collaborative writing. The analysis of the data gathered by observations of group interactions, interviews, and student texts reveals three patterns of engagement changes: from active collaborators to 'team escapers', from a supporter to a 'free rider', and unchanged dedicated writers. The findings are discussed concerning the factors that could account for the engagement changes, including students' language proficiency and learning needs as the individual factors and activity type as the contextual factor.