Abstract: Abstract: The contact angle hysteresis is one of the main causes of soil water characteristic curve (SWCC) hysteresis in unsaturated soil, the hysteresis of SWCC is a remarkable feature of soil under wetting and drying cycles. In this study, we prepared soil samples with different contact angles by adding repellent agents (octadecylamine) and found that the more the content of the water repellent agents, the greater the contact angle between the soil particles and the pore water. Contact angle measurement test were designed at the same time, drying-wetting cycles of soil were reflected in the test. Droplet 1 was the initial state of the droplet, droplet 2 was a little drop formed after droplet 1 evaporation immediately, and droplet 3 was a little drop formed 24 hours after droplet 2 evaporation. The test results showed that, the contact angles of sand were greater than the clay with the same water repellent agents content, this was mainly because of the geometrical shapes of the soil particles. In all of the soil samples, the contact angles of soil particles and the pore water decreased continuously during the evaporation process, until the contact angle became 0. At this time, the droplets were completely vaporized. The change of the contact angle on clay surface had a linear relation with time, but it was a nonlinear on sand surface. In the same samples, the evaporation time of droplet 1 was longer than droplet 2 and droplet 3 with the same liquid volume, the time spent in droplet 2 and droplet 3 complete evaporation was relatively close, but droplet 3 took a little more time to evaporation than droplet 2, this meant initial contact angle may affect the evaporation velocity of the soil. The larger the initial contact angle, the longer the evaporation time of the droplet would be. Either sand or clay, the contact angle of droplet 1 was greater than droplet 2 and droplet 3. This was because that the contact angle of droplet 1 was measured on the fully dry sample surface. The contact angle of droplet 2 was measured on the sample surface when soil particles were covered by water films from the droplet 1. The contact angle of droplet 3 was measured on the state in which the free water was evaporated completely. The initial contact angle of the three droplets reflected the contact angle of the soil under different initial conditions of wetting drying cycles. The water films of soil particles which the droplet 1 correspond to were the thinnest and droplet 2 correspond to be the thickest. There were three stages of droplet evaporation on a continuous solid surface：Initial stage, pinning stage and slipping stage. The evaporation mode of soil particles only existed in the initial stage and pinning stage, and there was no slip stage. That was, no matter how lager the initial soil contact angle was, by the end of the droplet evaporation, contact angle would be 0. In the whole evaporation process, the three-phase contact line did not move, and the solid-liquid area remained unchanged. The experiment confirmed that it was reasonable to suppose the contact angle was zero during soil drying, the contact angle hysteresis was one of the reasons why SWCC hysteresis loops became small during wetting-drying cycles.