Based on the importance of polymorphic regulation, the molecular mechanism of nucleation has been widely concerned. This review begins by introducing the development and limitations of nucleation theory for organic small molecule crystals, followed by a summary of the general research mentality adopted by current researchers. Moreover, the progress of the molecular mechanism of polymorphic nucleation and its application to the regulation of crystal forms are discussed. In addition, the development of scientific tools for the study of the molecular mechanism of polymorphic nucleation is also summarized, including experimental characterization and computational simulation, providing reference for relevant researchers. Finally, according to the main defects of current research and research ideas, research models and development directions of prospects and recommendations are put forward.