In order to effectively analyze the effect of terminal velocity of raindrops on soil erosion, the Ansys/Fluent software and VOF model were used to track raindrops freely, and the simulation of mixing rain and thin layer of water was conducted to analyze velocity changes of raindrops and the connection between velocity, raindrops diameter and height when raindrops fall to the surface of the water. The simulation results were verified based on relevant literature data. Results show that when the raindrops fall with a same diameter, along with the increase of height, the diameter of raindrops entering the water showed a trend of decrease, decreasing from the diameter of 15 m to 20 m to 40% of the original diameter.Under the condition of the same height, with the increase of diameter, the diameter of raindrops entering the water showed a trend of increase, increasing by 20% from diameter of 3 mm to 4 mm . When raindrops diameter was within 2 mm to 4 mm, and water layer thickness was 2.5 cm, raindrops hiting power disappeared.