Micro-teaching at universities and student teaching in secondary schools are standard forms of practice training for pre-service mathematics teachers in Chinese university teacher education programs. The former is guided by university professors, and the latter is guided by school teachers. In recent years, a special kind of micro-teaching practice held at university and guided by expert secondary mathematics teachers was organized to link the regular micro-teaching and student teaching. The purpose of this study is to investigate one aspect of the impact of micro-teaching guided by expert secondary mathematics teachers on pre-service teachers' teaching practice: learning to attend to precision. Four normal university students majoring in mathematics were selected and observed when they were teaching: before, during, and after the activity. Each lesson was videotaped, transcribed, and then analyzed using qualitative data analysis methods. The reflections from the participants were also collected and analyzed. The results indicate that micro-teaching guided by expert secondary mathematics teachers can help pre-service teachers attend to precision in their classroom teaching, but some lack of precision in performance was still observed in the pre-service teachers' lessons. The guidance and the exemplary lessons of expert teachers played an important role in changing the pre-service teachers' performance. The findings have the potential to be useful for mathematics teacher preparation and professional development programs. It could possibly also contribute to other research areas in mathematics classroom teaching.