How did Western race science and the concept of "race-mixing" shape modern Chinese identity, state-building, and governance? To Mix or Not to Mix explores the dramatic shift in perceptions of ethnicity, citizenship, and human bodies in China from 1895 to 1949. Drawing on rare archival materials—including medical journals, police files, and immigration records—it reveals how debates over miscegenation reflected the rise of racial-scientific thought and its integration into Chinese nation-building policies. By centering racial hybridity, this study reconstructs the intersection of nationalism, eugenics, and global intellectual currents, offering a fresh lens on modern China's sociopolitical transformations and its place in global history.