Based on unsaturated soil water movement theory, a three-dimensional mathematical model was developed to describe water flow movement under gravity subsurface drip irrigation (SDI). The effects of water supply pressure and dripper aperture on the soil wetting pattern, emitter charge and soil water potential with four typical soils from different subarea in Loess Plateau under gravity SDI were simulated and analyzed. The results indicate that the model can be used to analyze gravity SDI soil water infiltration characteristics and wetting pattern accurately. Gravity SDI is not suitable for Yulin area because of great deep leakage. Under the same irrigation quantity, pressure head and emitter diameter have slight effects on the wetting pattern but obvious effects on emitter effluent discharge in Ansai Sandy Loam, Luochuan Medium Loam and Wugong Heavy Loam. So the emitter aperture and water pressure can be decided only based on the field length and pipe infiltration loss for gravity SDI design. Meanwhile, it is necessary to adopt lower water pressure to reduce water supply pool height and the gravity SDI engineering investment. Especially for Wugong Heavy Loam, larger diameter dripper and higher water pressure will make the soil near drippers quick saturated and harden, therefore it should adopt the smaller diameter dripper and lower water pressure.