In the post-COVID-19 era，with the guidance of national policies and technical support，online education in universities is increasingly adopted by undergraduates. While enjoying the convenience，enrichment and progress of online learning，whether learning effect can be guaranteed is the common theme to be solved by all parts of online education. The achievement of learning effect requires not only the promotion of universities，teachers and platforms，but also the improvement of undergraduates’ online learning ability. Through the literature research，in-depth interview and questionnaire survey，this study constructs online-learning ability index of undergraduates from the perspective of digital literacy，and applies the index to investigate the development of Xiamen undergraduates’ online learning and digital literacy，and analyzes the relationship between them. And this study will give the promotion strategies for developing the online education in universities.