Most of current Chinese teachers' in-service learning takes place in two modes: listening to experts' lectures, and sharing experiences among peers. How a boundary-crossing learning context can be created so as to inspire teachers' deep reflection on their mindsets and change their routine behaviour remains a challenge. This paper explores how the teachers in a Chinese secondary school learned in a boundary-crossing lesson study with the support of outside scholars. Driven by the contradictions of value conflict between education for exams and education for student development, the teachers volunteered to participate in the project. Taking student group learning as the boundary object, the teachers retooled relevant theories and methods, improved teaching and learning, and generated new practical knowledge. The study identified four mechanisms that substantiated the teachers' boundary-crossing learning: negotiation of meaning, relationship construction, perspective transformation, and hybridization of practice. The findings of this paper will benefit teachers, teacher researchers and educators as well as policy makers by providing experiences of an alternative approach for teacher learning.