Group preparation for teaching contest, or lesson polishing, is a teacher professional development activity unique to China. Through participant observation and discourse analysis of a typical case, this study explores how a science lesson evolved through lesson-polishing process and how such process influenced individual learning and the development of local teaching community. Our work illustrates both the values and the issues of lesson polishing as a type of teacher professional development activity. On one hand, combining professional interactions and trial lessons, lesson-polishing activity opens up space for critical yet cooperative professional interactions and tryouts of different designs and teaching strategies, providing opportunities for individual learning and development of practical rationalities within local community. On the other hand, the functions of such activities are greatly limited by the tendency of refining every detail in lesson design, the existence of overriding dispositions and authorities with overriding power, as well as the focus on practical suggestions that can be directly implemented. Suggestions for improvement are made in the final discussion.