To understand the differences and changes of diurnal phreatic evaporation, the lysimeters at the Wudaogou hydrologicalexperimental station were applied to test in bare land and planting wheat or corn in lime concretion black soil and fluvo-aquicsoil. The results show that diurnal evaporation varies obviously at different growth stages when there are planting crops. When thewater demand is large, the value of phreatic evaporation is great. Two kinds of soils both show that the value in daytime is greaterthan that at night. At the initial growth stage, water requirements of crop are small, but the temperature in daytime is higher thanthat at night. In the early growth stage of wheat, the temperature is low in winter and diurnal difference of black soil is notobvious while that of fluvo-aquic soil is obvious at night. In bare land, fluvo-aquic soil is greatly affected by condensation of watervapor and breakage of daytime capillaries, and the value at night is greater than that in daytime. However, black soil is in verse.