Scholarly attention to foreign language learning boredom (FLLB) has surged in recent years. However, little is known about L2 learners' online FLLB experience prompted by the COVID-19 pandemic. To fill this gap, the present mixed-methods study explores the conceptual structure and sources of FLLB in an online learning environment. A corpus of 348 Chinese as second language (CSL) learners participated in a questionnaire survey and 10 of them attended follow-up interviews. Exploratory factor analysis revealed a three-factor structure underlying CSL learners' FLLB: classroom boredom, content boredom and teacher/learner boredom. Qualitative data enriched our understanding of the three-factor construct of FLLB and boredom sources in online classes. Results were discussed with reliance on the control-value theory of achievement emotions, previous findings, as well as their theoretical and practical implications for L2 teaching and learning. The newly found three-factor structure coincides with the long-lasting '3T' difficulties ['3T' difficulties refer to three major concerns in teaching CSL. They are connected with how to implement effective teaching strategies, develop high-quality teaching materials and build qualified and capable teaching teams] in International Chinese education; therefore, it inspires CSL researchers and teachers to attach more importance to the negative emotion of FLLB in the future.