Provided is a method of measuring and calculating distribution of the water temperatures of a river channel below a dam of a reservoir. Actually-measured data of the weather and the hydrology are combined, changes of the water temperatures along the river channel below the dam of the reservoir are measured and calculated, a position below the dam of the reservoir serves as the first measuring and calculating section, and the spacing distance of sections along the river channel is calculated according to an integral multiple of the one-day and one-night flow path of a river reach with the mean velocity. Heat exchange of water in the river channel is fully considered, detailed analysis does not need to be carried out on hydrodynamic processes, the mean velocity is used for replacing hydrodynamic calculation, the analysis of hydrodynamic equations required by numerical simulation calculation can be avoided, and workloads of calculation are reduced greatly. Meanwhile, according to the method, main factors leading to the changes of the water temperatures of the river channel are fully considered, and accuracy of calculation is guaranteed. As for the method, the good accuracy is obtained, the amount of calculation is relatively small, complex boundary condition data are not needed, work efficiency can be improved greatly, application fields can be widened, the accuracy and usability can be taken into account better, and the method can effectively assist in making discharging schemes of the water temperatures and the flow quantity of the reservoir.