Facing the growing popularity of autonomous online learning, students require more agentic engagement (AE) to construct a supportive learning environment for themselves. Existing studies based on self-determination theory (SDT) state that teachers' motivating style can predict students' engagement, and students' competence beliefs can moderate the predicting effect. However, few studies have discussed the correlation between teaching motivating styles and AE in middle- and high-school online courses, considering the students' self-regulated learning (SRL) ability as a moderator. To address this gap, this study examined the effect of autonomy-supportive motivating style (ASMS) and controlling motivating style (CMS) on AE, taking into account students' SRL ability as a potential moderator. A total of 425 valid questionnaires were collected to evaluate the model. The results revealed that teachers' ASMS positively predicted students' AE; CMS positively predicted students' AE; and SRL ability positively predicted AE. However, SRL ability did not significantly moderate the correlation between teachers' motivating styles and AE. These findings provide educators with insights into how to improve students' AE, and suggest that teachers should be involved as much as possible in online teaching interactions, and explore strategies to enhance the effectiveness of students' AE.