Abstract This research used an integrated model to study how learners' cognition, motivation, and self-regulation jointly promoted learners' high-quality collaborative English writing. Participants were 10 junior students (8 females and 2 males) in a Chinese university, who were asked to write collaboratively in English as a part of the formative assessment of their educational course. Multiple sources of data were collected and analyzed, including in-depth interviews, students' face-to-face and online group interaction records, course materials, etc. Interpretive phenomenology analysis was adopted for analyzing the data. Based on the analysis, four findings emerged. Specifically, learners' use of cognitive strategies, including repetition, elaboration, organization, and self-reflection strategies in individual and collaborative learning promoted their writing quality. With a better understanding of their collaborative writing motivation (task-oriented and learning-oriented), conditions for its internalization (value of task; teacher and peer supports), and life motivation, learners gained writing progress from motivational perspective. The application of a series of self-regulated learning strategies, including time management, environment management, emotion management, and help-seeking, enhanced students' high-quality writing from self-regulation perspective. Overall, an integrated collaborative model that incorporated cognition, motivation, and self-regulation was applied to deal with barriers to collaboration effectiveness and comprehensively promoted learners' writing quality through knowledge, idea, and skill enhancement. Finally, these findings were discussed with implications provided to future implementation and research of collaborative writing.