This article aimed to explore which factors caused Thai learners' anxiety in an online Chinese course provided by Chinese universities, how Chinese proficiency affected online learning anxiety, and how online learning anxiety influences learning engagement. The participants, i.e., learners of Chinese as a foreign language (CFL), completed the questionnaires dealing with online learning anxiety and learning engagement, and their Chinese proficiency was measured by the Hanyu Shuiping Kaoshi (HSK) level 4 test. A semi-structured interview was conducted to enrich the study and the full picture of learner anxiety and engagement in online learning. The result of the questionnaires showed that the level of CFL learners' anxiety was moderate. The crucial factor in anxiety was the fear of communication in Chinese. ANOVA analysis indicated that Chinese proficiency also affected CFL online learning anxiety; most learners of low Chinese proficiency had a higher level of anxiety than those with moderate and high Chinese proficiency. Additionally, online engagement in the dimension of social presence was significantly related to CFL learners' anxiety. The interviews also revealed that classroom interaction was the crucial element of online learning, which may lessen learners' anxiety and increase their engagement.