Due to its outstanding advantages of openness, unrestraint and low cost, MOOCs have been growing rapidly in recent years. However, this new self-directed online learning model faces the challenge of the low course completion rate. Based on self-determination theory, this paper combines learners' intrinsic motivation and extrinsic motivation, and thus constructs a comprehensive theoretical framework to explore the learner engagement mechanism in MOOCs learning. Empirical results reveal that learners' intrinsic motivation (interest) and extrinsic motivation (perceived knowledge value) is critical to learner engagement, and thus improve learner satisfaction. However, the self-development motivation of extrinsic motivation has no significant effect on learner engagement behavior. The findings enrich and expand the relevant researches on learner engagement and e-learning, and provide guidance for practitioners.