This study investigated the sustainable development of university EFL learners regarding their engagement, satisfaction, and self-efficacy in online learning environments during the outbreak of COVID-19. In a questionnaire survey with a sample of 428 Chinese undergraduate EFL learners, the students reported a favorable view of online learning environments and subjective learning outcomes. Behavioral engagement was positively related to involvement. Emotional engagement was positively related to student cohesiveness and negatively related to teacher support. Satisfaction was not related to any of the learning environment factors. Self-efficacy mediated the effect of student cohesiveness and student involvement on behavioral engagement, emotional engagement, and satisfaction. These results of the study have implications for creating a sustainable online learning environment and promoting EFL learners’ sustainable development.