Abstract: Hydrological similarity theory is the core research topic of comparative hydrology and an important theoretical basis for obtaining hydrological model parameters in ungauged basins. Based on the analysis of the origin and development of the concept of hydrological similarity, two basic problems of discriminant condition and applicable condition in the theory of hydrological similarity are put forward. This paper reviews the progress of hydrological similarity theory from three aspects: classification framework, hydrological similarity index and hydrological similarity evaluation, and summarizes the application of hydrological similarity theory. Finally, based on the current progress and shortcomings, three key issues are pointed out: the definition of hydrological similarity, the discriminant basis and applicable conditions of hydrological similarity, and the influence of the hydrological similarity index and hydrological similarity degree on the selection of similar basin. This research can provide a theoretical basis for hydrological forecast of ungauged basins, exploration of hydrological physical law and improvement of hydrologic similarity theory.