Due to the impact of COVID-19, most Chinese universities have launched livestream teaching. Faced with this significant change of teaching mode, teachers experienced different emotions, including predominant negative emotions such as anxiety, stress, and anger, alongside a few positive emotions like satisfaction, love, and happiness. With the rising attention on teacher emotion research, this study explores the emotional experiences of five Chinese English as a foreign language (EFL) teachers in livestream teaching. Drawing from data collected via interviews and case documents, it examines the causes of these teachers’ emotions and the impact thereof on their sustainable professional development. The findings suggest that teacher emotions were produced through the interaction between teachers’ goals and the environment, which included students’ performance, features of livestream teaching, and the institutional livestream teaching requirements. The impacts of teacher emotions on their sustainable professional development were identified: deepening teachers’ understanding of online teaching, shaping teacher identities, and motivating teachers to take action. Implications regarding developing teachers’ coping strategies for various emotions and sustaining their professional development in online teaching are also included.