[Objective] Soil evaporation under poplar shelterbelts was investigated, and the in situ soil evaporation estimating method was developed, in order to provide support for water cycle pattern and efficient water use research in forestry.[Methods] Soil evaporation and water surface evaporation under the forest were measured by micro-lysimeters and the standard 20 cm evaporation pan. Reference water temperature in the pan and soil temperatures were also measured. Then the relative evaporation (RE), the ratio of soil evaporation to pan evaporation, and the relative temperature difference (RT), the ratio of temperature difference between soil and water temperature to water temperature were calculated.[Results] The solar radiation was the major factor for the evaporations of the under-forest soil and water surface. Air temperature, humidity, and wind speed have poor correlation with soil evaporation because of the low soil water content. The relationship between RE and RT could be described with a two-stage function, RE decreased with the RT increasing for RT being less than 0.11, after that the RE approached a constant of 0.164, indicating a water vapor diffusion process.[Conclusion] Under poplar shelterbelts, RE decreases gradually with the growth of RT and eventually tends to be a constant. The developed method was validated using an independent data set with relative error of less than 2%, indicating an excellent predication of soil evaporation. Therefore, this study provides a new method for the estimation of soil evaporation and the developed method can be used to estimate soil evaporation.