Soil moisture's movement, redistribution and spatial-temporal stability under the precision drip irrigation are different from the traditional ground irrigation, and the drainage process and displacement are also different. In order to simulate the farmland drainage process under the precision drip irrigation accurately, this paper sets up a mathematic model based on the Richards equation and the finite difference method according to the distribution characteristics of soil moisture. Then the model was identified and verified through field observation of farmland drainage process under drip irrigation. The results showed that the groundwater level of the farmland gradually becomes steady as time goes on in the irrigation cycle without rainfall, with an absolute errors range in -13~21 mm between the simulated and measured values. In the simulation of farmland drainage process after rainfall, the absolute errors between the simulated and measured values of the NO101, NO501 and NO505 observation holes are -27~47 mm, -27~-76 mm and -27~-79 mm, respectively. The trend of the simulated values agreed well with the measured values in time and space. The model can reflect the drainage process of farmland under the precision drip irrigation well, and can provide reference for information management of large irrigated area and design of drainage system under the precision drip irrigation.