This study examined the dynamic and complex development of college students' engagement in an online English for academic purposes (EAP) course. The study adopted a case study design, collecting data from multiple sources such as class observations and student interviews. Data analysis revealed the learners' engagement profiles and patterns throughout the duration of the course, highlighting the interactions and transactions between learners and the learning environment. In this online learning environment, the behavioral, cognitive, emotional, and social dimensions of learner engagement underwent dynamic development through interactions between learners and contextual complexity. The features and developmental trajectories of learner engagement were influenced by a series of language, cultural, course, technical, and individual factors, such as full English instruction, cultural differences in educational philosophy, course mode, technical support, learners' language proficiency, and motivation. These findings shed light on learner engagement in an online EAP learning context and have the potential to enrich our understanding of the multifaceted, situated, and dynamic nature of engagement. The study has implications for language pedagogy, curriculum design, and program innovation in a globalized world.