The expansive soil is sensitive to climate. In this study,the indoor and outdoor evaporation experiments on Pingdingshan expansive soil with various initial water contents have been studied. The variation of soil water content with depth in the evaporation process has been obtained. The evaporation intensity is different between indoor and outdoor soil specimens,but their evaporation coefficient is less different. The evaporation coefficient has correlation with soil water content and water content gradient. When the surface soil is close to saturation, the evaporation coefficient fluctuates around one. With continuous evaporation,the evaporation coefficient decreases with the surface water content,and the water content gradient increases with the decrease of surface water content. When the surface water content reaches the residual water content,the water content gradient reaches the maximum. The continuous evaporation water content gradient will gradually decrease. A good logarithmic relationship between the evaporation coefficient of soil and the evaporation duration can be seen.