Purpose: Lesson study (LS) is generally regarded as a pathway for teachers' professional development and a method for teachers' instructional research. LS has been regarded as having the potential to drive large-scale reform but little is known about how it does so from a district level. Therefore, this paper aims to reveal how lesson study promote district education reform. Design/methodology/approach: This study offers an in-depth case study of how District Y of Shanghai, China, took LS as the primary method in promoting its District Project of Building Curriculum Leadership in Schools. By analyzing the key project documents and achievements in project promotion, and interviews with the major Project leaders at District and school levels, this study explored the practice and impact of LS as a tool to promote district reform. Findings: In the District Project, LS has been a medium to address each individual school's real problems of practice and turn them into reform vision and reform will in alignment with District goals. Five levels of school curriculum texts have been planned, designed, translated, implemented, reflected on, updated and mutually adjusted systematically through LS to ensure consistency in transforming District reform vision into classroom practice. Different models of teaching-research community building were found in sampled project schools and professional expertise was built with district support to promote reform. The curriculum leadership development through LS has shaped reform leader schools and formed a collection of LS exemplars circulated in schools as high-quality curriculum packages, which laid the foundation for district-wide reform. Originality/value: The innovative practice of LS in China's education reform has expanded its reach from within one classroom to the entire district curriculum system and made it an important tool to drive large-school district-based education reform.