This paper reports the findings pertinent to the motivational dynamics in learning English for Academic Purposes (EAP), which are a part of a large-scale mixed-methods study. The study was aimed to explore the adaptation of a cohort of Chinese tertiary students when transiting to an English-as-Medium-of-Instruction (EMI) university from Chinese-medium schools in China. Data sources include a questionnaire survey and semi-structured interviews over a 10-month length of time. Analysis of the quantitative data revealed that the participants became much more strengthened in the overall and dimensional level of motivation after being engaged into EAP learning at the EMI context for two semesters. The qualitative results indicated that the consolidation of the participants' motivation might result from the newly mediational roles of socio-cultural elements at the EMI environment pertaining to the learning discourses, cultural artifacts, important others and the study of content subjects with the target language. This study crystallizes the situationally constructed disposition of second language (L2) motivation which is responsive to context. The pedagogical implications for EAP teaching are also discussed.