Teacher professional learning is shaped by multiple contexts in a complex way. Previous studies mainly focused on teacher learning in school-based contexts, and rarely explored how teachers learn across schools and in other situations. Adopting the framework of boundary crossing learning, this study examined the processes of teachers' professional learning when they participated in Master Teacher Studios in mainland China. Through the qualitative case study approach, this study summarised four learning mechanisms: seeking common ground and reserving differences, growing through formal and informal coordination, exposing the gap and reflecting one's limits, and transforming practices that incorporate one's teaching 'soul'. Further, intrapersonal, interpersonal and institutional factors that contribute to teacher learning as boundary crossing are discussed.