AbstractThe burgeoning English‐medium instruction (EMI) programs have aroused worldwide scholarly interest in giving language support for EMI subject teachers and their identity‐informed professional development. This study explored how an EMI subject teacher's identities and emotions were intricately interlinked before and after his co‐teaching practice with an English as a foreign language (EFL) teacher at a university in Midwestern China. Ethnographic tools including interviews, observations, and small talks were used over a 10‐month collaborative program. The findings show that the participant teacher's identities and emotions were interdependent with each other and mediated by collaborative teaching practice. Futhermore, emotional attachment to English enhancement and deep engagement in co‐teaching contributed to (re)construction of teacher identities as a content teacher, a competent English user, and an English learning facilitator. The participant's emotional attachment was explicated by his perezhivanie, or the emotional experience of his previous English learning and the emergent EMI teaching contexts. The study highlights the primacy of reconstructing teacher identities and supporting teacher emotional experiences in EMI teacher training and education programs.