This study aimed to explore the effect of academic self-efficacy on online learning engagement and its underlying mechanism. A total of 310 colleges students were surveyed with the Academic Self-efficacy Scale, Learning Motivation Scale, Flow Experience Scale and Online Learning Engagement Scale. The results were as follows: 1) Academic self-efficacy could significantly predict student online learning engagement. 2) Learning motivation and flow experience respectively played significant mediating roles between academic self-efficacy and online learning engagement. 3) Learning motivation and flow experience played significant chain mediating roles between academic self-efficacy and online learning engagement.