There has been an increase in high school programmes utilizing English as a Medium of Instruction (EMI) in China. Considering the challenges posed by the rapid growth of these programmes and the scarcity of qualified teachers and appropriate curriculum resources, this study investigated the implementation of inquiry science teaching in EMI science classrooms. Through classroom observations, semi-structured interviews and a collection of class materials such as course syllabi, lesson plans and lecture handouts, this study examined the words and actions of five science teachers aiming to understand the design and practice of inquiry science teaching in EMI settings. Thematic analysis of the qualitative data suggested that teachers attempted to develop students’ understanding of technical vocabulary and prioritized English language teaching objectives related to academic vocabulary acquisition when designing and teaching. Furthermore, teachers drew on students’ first language (L1) in scientific practices that required them to use high-order thinking skills. However, teachers’ use of L1 reveals several issues, including the tension between teachers’ linguistic ideology in EMI policy and translanguaging pedagogy, and the reliance on L1 in translanguaging but the lack of semiotic resources. These findings have important implications for science teaching in EMI context, especially inquiry science teaching.