The invention discloses a selection method of the time scale of incoming flow material of a reservoir to be built. The selection method includes the steps that material of similar reservoirs, with known detailed incoming flow material, in the same climatic region is collected, and the collected incoming flow material is generalized; taking the generalized incoming flow material as boundary conditions of water temperature numerical simulation calculation of the reservoir, water temperature is calculated; the steps such as extraction and correlation analysis are carried out on simulated water temperature distribution results in the cared regions; according to the principle that the influences on water temperature distribution of the reservoir by incoming flow boundaries between the similar reservoirs in the same climatic region are basically the same, the time scale of the incoming flow material of the reservoir to be built is finally determined. According to the principle that the influences on water temperature distribution of the reservoir by incoming flow boundaries between the similar reservoirs in the same climatic region are basically similar, the maximum time scale, needed by accurately simulating water temperature distribution in the cared regions of the reservoir to be built (the reservoir with the detailed incoming flow material unknown), of the incoming flow material is concluded, both the economy and the accuracy of reservoir water temperature simulation results can be considered, and the selection method is beneficial for utilization and popularization.