The establishment of initial seepage field close to the real state is an important prerequisite for the correct analysis of slope rainfall infiltration. In view that methods of initializing saturated-unsaturated seepage field in FLAC^3D are rarely reported, in this paper we put forward and introduce in detail the setup method of initializing saturated-unsaturated seepage field. According to the five distribution forms of matric suction, two initialization methods are proposed corresponding to the linear and non-linear distribution laws of matric suction. For the linear distribution of matric suction, MIDAS modeling and FLAC^3D built-in FISH language are employed to initialize the seepage field. For non-linear distribution, since it is difficult to represent the complex geometry of groundwater level surface by using function, the groundwater level is simplified into a plane for the initial setting of seepage field by using FISH programming language. Comparison with conventional initialization method demonstrated that the initial seepage field has a great influence on the final calculation result of slope rainfall infiltration. When the initial seepage field is closer to the real state, the calculation accuracy is higher.