This paper pointed out that local water resources carrying capacity should be taken as the prerequisite for drought study. The hydrological knowledge was used to analyze the characteristics of the nature water cycle, and revealed that the water resources carrying capacity how to develop drought mechanism in recession process of land surface evaporation and runoff. Moreover, it de - fined the generalized drought, summarized the relation and distinction between the special droughts in meteorology, agriculture and etc., emphasized that the hydrology is the only subject to give scientific and rigorous definition for drought.