Although previous studies have examined the impact of online formative assessment on second language learners' writing development, scant attention has been paid to how writing teachers engage with online formative assessment and the influencing factors. By exploring three English-as-a-foreign-language (EFL) writing teachers' engagement with online formative assessment during COVID-19 in three universities in China, the qualitative case study identified three types of teacher engagement: disturbing, auxiliary, and integral, embodied by their varied emotional, physical-cognitive, and social investment in the formative use of information communication and technology in writing assessment. These individualized engagements were primarily influenced by teachers' beliefs, digital literacies, and learning/teaching experiences, and mediated by relevant contextual and technological factors. Implications are provided for supporting EFL writing teachers' engagement with online formative assessment.