As an essential community of practice which could facilitate teachers' professional development and enrich research in teacher education, the teacher--researcher partnership (TRP) has not been sufficiently investigated in China. As reported in this paper, we conducted an empirical study on TRPs between secondary school mathematics teachers and university researchers by applying activity theory to investigate their collaboration and contradictions, through the process of co-designing lesson studies. We found four levels of contradictions, namely, primary, secondary, tertiary, and quaternary, within which the fourth one occurs less frequently compared to the other three. To ameliorate or even resolve the contradictions, it requires both sides to (1) achieve agreement on understanding the objects, (2) promote equality in communication and mutual respect, (3) facilitate transformation in identity, and (4) construct a united community. In addition, the investigation provided a depiction of how teachers learn and grow in the TRP as well as how teachers and researchers interact in the partnership, which in turn enriches both teacher education and activity theory.