The invention discloses a method for determining the temperature boundary of a bottom layer in soil hydrothermal coupling simulation, and relates to the technical field of water circulation in cold regions. According to the method, the influence of air temperature on the bottom soil temperature is considered, and after the soil temperature boundary changing along with time is adopted, the method can be suitable for simulating the soil temperature when the temperature of the bottom layer changes greatly along with time; The change of the bottom temperature of the calculation layer along with the temperature and the upper temperature is fully considered, and the calculation accuracy of hydrothermal coupling calculation is improved. Furthermore, soil temperature boundaries are set according to the calculation units; The method can be suitable for basin hydrological models of various scales, calculation boundaries can be specifically set according to soil temperatures of different basins of various regions, the problem that in an original hydrological model, changes of temperature boundaries of bottom layers in partitions are large can be effectively solved, and the calculation precision is improved while the parameter setting amount is reduced.