While there has been much research into teachers’ professional quality in general education, scant attention has been paid to the professional quality of EFL (English as a foreign language) teachers, especially those working in adverse areas, for their sustainable career trajectories. Drawing on Wu’s framework of EFL teachers’ professional quality, this study adopted a mixed-method study design to investigate the professional quality of experienced EFL teachers in China’s rural areas. A questionnaire was administered online to 367 Chinese EFL teachers from 15 high schools in the northeast part of China. Follow-up interviews were then conducted with four volunteers. The results of the exploratory factor analysis produced a two-module structure of teachers’ professional quality, addressing teacher beliefs and English language pedagogical content knowledge (PCK) for their sustainable career development. The teacher belief module addresses beliefs about language teacher learning and development as well as beliefs about foreign language teaching and learning. The English language PCK module includes two subscales: English teaching capacity and English knowledge and skills. Descriptive analysis showed that these participants had high levels of professional quality. Possible explanations for the interpretations of EFL teachers’ professional quality are provided from the qualitative data. Based on the above findings, the paper offers some implications for sustaining EFL teacher development in the future.