As a very complex and giant system, the hydrology and water resources system has a close relationship with the economic-social system and ecological environment system, and they jointly form a complex system. Therefore, the research of the hydrology and water resources system requires very extensive theories, technology and methods. Among them, the quantitative research method of harmony theory based on the harmony degree equation plays a very important role in solving the problem of hydrology and water resources. The proposed process of the quantitative research method of harmony theory was reviewed, and the research demands in the field of hydrology and water resources and the harmony theory application were analyzed in this paper. Based on the summary of the previous researches, the application prospects of harmony degree equation in the field of hydrology and water resources were detailed mainly from three aspects, i.e. the systemic analysis computation, the comprehensive evaluation and the systemic decision analysis. The harmony degree equation (HDE) evaluation method, the quantit-ative coordination degree method based on the unity degree (a), the quantitative opposition situation method based on the difference degree (b), and the harmony equilibrium model based on the HDE have been proposed.