Learners' anxiety and emotional engagement have attracted a surge of scholarly interest due to their considerable relevance to students' academic performance and psychological well-being. However, the two variables await more exploration in the English as a Foreign Language (EFL) context. To fill this gap, we aimed to investigate these two variables in online and offline English-speaking classes, respectively, at a university in China. We adopted a sequential mixed-methods research approach. First, we used questionnaires to measure and compare the level of anxiety and emotional engagement in the two classes. Then, we conducted semi-structured interviews to gain a deeper understanding of the factors affecting learners' online and offline learning experiences. Quantitative results showed that students had higher levels of anxiety and lower levels of emotional engagement in the online environment than in the offline context. The subsequent qualitative findings revealed a complexity of positive and negative factors affecting anxiety and emotional engagement in the two instructional contexts. Our findings can shed light on research and practices concerning learner emotions in online and offline EFL instruction. Strategies were also provided to help teachers mitigate learners' anxiety and enhance their emotional engagement, which includes formulating a learning community, providing emotional support, alleviating students' technical stress and distraction, and incorporating group-based interactive activities into the course.