Online learning has gradually become the new mainstream learning norm during the post-epidemic era. To ensure college students’ online learning effectiveness, they need to be proactively engaged in their online learning, which means that they need to maintain a high level of agentic engagement. However, it is not clear what factors influence college students’ agentic engagement in online learning environments. According to self-determination theory (SDT), the teacher, as an important external factor, can influence students’ learning engagement and learning effectiveness. Meanwhile, self-regulated learning (SRL) ability is important for college students in online learning. Based on existing theoretical foundations, therefore, the current study attempts to explore the following research questions: Are there relationships between teachers’ motivating styles (including autonomy-supportive style and controlling style), college students’ online SRL ability, and college students’ online learning agentic engagement? If yes, how does the former influence college students’ online learning agentic engagement? To respond to these research questions, random sampling was used to collect a total of 681 valid data from college students with experience in online learning. Then, a pilot test, exploratory factor analysis, and confirmatory factor analysis were first conducted to confirm the reliability and validity of the data. Correlational and mediating analyses were then conducted using SPSS 21.0. According to the results, firstly, teachers’ motivating styles (autonomy-supportive style and controlling style) and college students’ online SRL ability were positively correlated with college students’ online agentic engagement. Secondly, teachers’ autonomy-supportive style was positively correlated with college students’ online SRL ability, but the controlling style was negatively correlated with their online SRL ability. What’s more, college students’ online SRL ability, as a mediating factor, acted as a partial mediation between autonomy-supportive style and college students’ online agentic engagement, and there was a suppressing effect of college students’ SRL ability between teachers’ controlling style and college students’ agentic engagement. The results imply that teachers should choose motivating styles appropriately based on students’ online learning characteristics and content, and college students should develop online SRL ability to improve their agentic engagement and ultimately achieve good online learning effectiveness.