Learning engagement is acknowledged as a vital proxy for evaluating the quality of online education, and teachers' emotional support, as an essential component of social support within the educational environment, positively influences learners' online learning engagement. The present study draws on Person-environment Interaction Model and Broaden-and-Build Theory to investigate the effect of teachers' emotional support on online learning engagement for English as a Foreign Language (EFL) learners and the mediating roles of academic emotions, specifically enjoyment and boredom. A questionnaire survey was conducted among 411 Chinese college EFL learners who were selected through convenience sampling. The results revealed that teachers' emotional support was not a direct predictor of online learning engagement but indirectly affected it through the mediating role of enjoyment and the serial mediating roles of both enjoyment and boredom. Additionally, the mediating effect of enjoyment was much stronger than the chain mediating effect of enjoyment and boredom. Teachers should pay more attention to facilitating learners' enjoyment and relieving their boredom, which would enhance the positive impact of teachers' emotional support on online learning engagement. These results have some pedagogical implications for improving students' online English learning engagement and enhancing the quality of sustainable online education.