This study examines how mathematics teaching research specialists mentor practicing teachers during post-lesson debriefs of a lesson study in China. Based on a systematic, fine-grained analysis of 107 h of videotaped mentoring meetings of 20 groups of teachers and teaching research specialists from different elementary schools, this study reveals that the Chinese teaching research specialists pay a great deal of attention to "practical knowledge" which consists of setting students' learning goals, designing instructional tasks, formative assessment of students' learning and improving instructional behaviors. Less attention is paid to mathematics content knowledge and general pedagogical knowledge. Meanwhile, the teaching research specialists tend to comment on lessons in general and address anticipated problems based on their previous experience, and pay less attention to address issues raised by the teachers or to engage in dynamic dialogue with them. On the basis of the data analysis, a framework for analyzing mentoring activities emerges. The strengths and weaknesses of the teaching research specialists' mentoring strategies are identified through the framework, and suggestions to improve the teaching research specialists' mentoring strategies are discussed.