As a growing phenomenon in higher education, English medium instruction (EMI) places high demands on content area teachers regarding linguistic proficiency, content knowledge, and pedagogical strategies. To improve EMI practice and facilitate EMI teachers’ continuing development, a focus on teacher expertise thus becomes imperative. Focusing on an intrinsic case, this study investigates the attributes of an EMI teacher’s expertise and its influencing factors in the field of law at a Chinese university. Drawing on data from semi-structured interviews, classroom observations, and artifacts, the study reveals three critical dimensions of the teacher’s expertise: 1) maximizing the power of language, 2) facilitating disciplinary socialization, and 3) empathizing with students in the EMI learning process. Meanwhile, the three attributes functioned in a continuous, reflective, and collaborative manner entangling with the teacher’s continuous learning, agentive reflection, and social interactions within the disciplinary community. The paper concludes with practical suggestions in relation to both EMI teaching and teacher development.