To promote English learning is an alleged benefit of implementing English-medium-instruction (EMI). However, English learning is supposed to occur incidentally in most EMI contexts. From the linguistic-ecological perspective, incidental language learning is premised on two conditions: linguistic inputs are perceived by learners as affordances, namely, perceived opportunities for action provided by the environment, and learners are excising their agency to engage with these affordances and the learning environment. Despite a substantial body of research investigating various aspects of EMI, there is a scarcity of research on students' perceptions of EMI classroom activities and teachers' instructional language use as English learning affordances, the manifestation of their agency and its mediating factors. Drawing on the ecological concepts of 'affordance' and 'agency', this study intends to fill this void using a mixed methods design. One EMI teacher and 134 students in a Chinese university were involved as participants. Data were collected primarily through classroom observation, a questionnaire, and interviews over a 2-month period. Findings show that activities that required students' active engagement proved to be affordances perceived more positively. The manifestation and development of English learning agency were varied and personalised, mediated by both contextual and individual factors. Implications are also discussed.