Collaborative writing instruction has been widely adopted in second language (L2) writing pedagogy, and the human-AI collaborative approach has recently gained increasing traction in this field. Despite the promising potential of this pedagogical model in facilitating L2 writing, the factors influencing learners' willingness to engage (WTE) in human-AI collaborative L2 writing remain underexplored. Based on the Stimulus-Organism-Response (S-O-R) model and the Technology Acceptance Model (TAM), this study explores how perceived teacher support contributes to EFL learners' WTE in human-AI collaborative L2 writing. A total of 503 Chinese university EFL learners with AI-aided L2 writing experience were included in this quantitative study. Findings from questionnaires and regression analyses indicate that perceived teacher support is directly and positively linked to the EFL learners' WTE. It also exerts an indirect association with WTE via the independent mediating roles of technology acceptance and task flow. Furthermore, technology acceptance and task flow jointly form a chain-mediating pathway between perceived teacher support and WTE. These findings not only broaden the application of the S-O-R model and TAM within human-AI collaborative writing contexts but also provide valuable implications for L2 writing pedagogy in AI-aided contexts.