Online learning has become the new educational pattern during the COVID-19 pandemic and is likely to supplement conventional schooling in the post-pandemic world. Lacking prior online learning experiences, the population of K-12 students deserves our special attention. Using purposeful sampling, this study investigated K-12 online learning experiences in China based on a large-scale survey (N = 118,589). Leveraging both quantitative and qualitative evidence, this study supported online learning as a flexible alternative to conventional schooling in emergency situations with a discussion of its benefits and limitations, and revealed key findings regarding K-12 students' online learning pattern, experiences, and engagement, as well as the influencing factors. The research findings can inform the future design and implementation of online learning programs in primary and secondary schools.