Background: Considerable evidence suggests that students' achievement emotions are important contributors to their learning and success online. It is, therefore, essential to understand and support students' emotional experiences to enhance online education, especially under the COVID-19 context. However, to date, very few studies have investigated how students' achievement emotions might be affected by teaching and learning factors in online learning environments. Objectives: Based on Pekrun's (2006) control-value theory of achievement emotions, this study examined the influence of students' perceived online teaching quality and appraisals of control and value on their achievement emotions in an online second language (L2) learning context instigated by the COVID-19. Methods: Data were collected using an online survey of 1503 undergraduates from ten universities across different regions of China. Confirmatory factor analyses (CFA) were performed to test the validity of the instruments and structural equation modelling were conducted to examine the relations between L2 teaching and learning variables. Results and Conclusions: Results revealed that perceived teacher internet and communication technology competence had positive effects on enjoyment, but negative effects on anxiety and boredom, and these effects were fully mediated by perceived control and/or value. Perceived chaotic teaching structure (CTS) had reverse effects for the three emotions. Perceived CTS effects on enjoyment and anxiety were fully and partially mediated by perceived control, respectively. Takeaways: Online teaching quality is critical for students' appraisals and emotions towards L2 learning. Teachers should strive to improve online teaching quality and design interventions targeting students' perceived control and value to temper their affective experiences associated with the crisis-prompted online teaching practices.