Metacognitive knowledge, closely correlated with one's writing skills, has received increasing research and pedagogical attention. Applying Flavell's metacognition framework, this study adopts a quasiexperimental design to investigate whether and how a DMC assignment as a pedagogical intervention influences Chinese English as a Foreign Language (EFL) writers' metacognition knowledge development. A total of 49 EFL college students were divided into the control (n = 25) and the experiment DMC (n = 24) groups. Both groups took pre- and post-intervention surveys on metacognitive knowledge. To understand how the development took place, we also collected and analyzed data on focal students' learning logs and post-assignment interviews, where students discussed their writing processes in detail. The results suggest that although both groups showed noticeable development in person and strategy knowledge, the DMC group exceeded the traditional writing group in the latter category, which could be attributed to the fact that students in the DMC group were engaged in more decision-making opportunities. This study provides valuable implications on designing DMC assignments in EFL writing classrooms to engage students in frequent reflection, conscious decision-making, and creative expression.