Formative assessment (FA) has enjoyed much popularity in educational assessment in recent years, especially for the teaching of writing. However, the application of FA in academic English writing (AEW) has rarely been studied. Employing both quantitative and qualitative methods, this study is designed to explore the effectiveness of FA in the process approach of AEW for EFL learners in China. Results from a T-test show that there is a significant difference in students' performance between the experimental and the control group. Meanwhile, the results of repeated measures ANOVA demonstrate significant difference among the three drafts produced by the participants in the experimental group. Qualitative studies also show that feedback provided by both peers and the teacher is treated seriously by most learners, which contributes to the gradual improvement of their writing. The results of the study suggest that FA may be more effective than summative assessment (SA) for the improvement of learners' AEW ability.