Online learning, from birth to now, has become a more dynamic research area. Online learning is a way of learning that student as the main body, teacher as the main leader, however, due to the space and time separation between teachers and students in online learning, it is difficult to grasp the level of student engagement in the learning process. "No engagement, no learning", even if there is a good learning condition, it cannot leave the students' engagement. Whether the learning is successful is closely related to whether students are engaged, how much they are engaged and how to engage in learning. Therefore, more and more researchers pay much attention on student engagement degree. In this paper, we analyze the current researches of student engagement in online learning, and emphasize the importance of student engagement and the necessity of further research in online learning.