This study investigated how peer instruction integrated with process writing influences EFL (English as a Foreign Language) students’ writing competence in a Chinese university. It randomly chose one intact class of non-English majors to implement the peer instruction and process writing approaches. The researcher divided the class into seven sub-groups to fulfill the writing tasks smoothly. Employing a qualitative research design, this study utilized observation methods to collect data. Three non-participant observers were invited with the researcher to observe the writing sessions among the sub-groups in the intact class. Thematic analysis of observation data showed that students effectively used peer instruction skills during all stages of process writing: planning, generating ideas, translating, editing, and revising. Integrating peer instruction with process writing served as a positive catalyst, motivating EFL students. Following Vygotsky’s Zone of Proximal Development (ZPD) principle, this integration significantly broadened students’ ZPD. By working within this zone, students unlocked their full developmental potential, surpassing their current abilities, leading to a more efficient and effective writing process.