Although research on teacher collaboration has proliferated in the last few decades, scant attention has been paid to the development of teacher collaboration in school contexts. Informed by the perspective of complexity theory, this study investigates the complex process of teacher collaboration through qualitative interviews in an English teaching research group (TRG) in a secondary school in China. The findings reveal three distinct stages in the development of teacher collaboration, labeled as 'breaking the ice,' 'everything is out of control' and 'learning how to collaborate through collaboration.' The study shows that teacher collaboration is an adaptive, complex system that evolves through internal self-organization and interaction with external stakeholders and systems, such as school management and university researchers. The study provides useful insights into the ways of facilitating and sustaining teachers' collaborative practices to enhance school effectiveness and improvement in specific educational contexts.