The COVID-19 pandemic has posed an unprecedented threat to the resilience of higher education worldwide, and university students had been thrust overnight into the emergency online learning. The end of COVID-19 pandemic is in sight, however, teachers and students have been forced to live with its disruptive effects indefinitely. Online teaching has been regarded as a panacea for maintaining the resilience of higher education during COVID-19 pandemic, it is significantly important to reflect on students' learning experiences and learning gains in online classes to improve future online teaching quality. Using a sample of 3514 participants located in the Chinese university context, this study aimed to investigate the determinants that affect students' learning gains in online learning anchored by self-determination theory. The results indicated a significant positive impact of teachers' emotional support in predicting students' learning gains in online classes, and also revealed the positive mediating effect of students' basic psychological needs satisfaction in bridging these two variables. Moreover, the findings highlighted the sophisticated role of students' achievement motivation interacting with teachers' emotional support to influence students' basic psychological needs satisfaction and learning gains. Implications regarding how to improve undergraduate students' learning gains in the future's online teaching were discussed.