Although the literature widely acknowledges the benefits of formative writing assessment in daily teaching, there is limited understanding of how frontline teachers implement it and the reasons behind their approach, particularly in EFL secondary school contexts where assessment has predominantly served selective purposes. Drawing on interviews, voice memos, document, and relevant artefacts, this study examined how a teacher implemented formative writing assessment and what influenced her decision-making process in a public secondary school in mainland China. The research findings shed light on how teachers working with beginner-level students can draw on formative writing assessment to develop students' L2 writing skills and on factors that might influence the realisation of formative writing assessment. The study also offers pedagogical recommendations for teachers and teacher educators and suggestions for future research.