Abstract: The groundwater is an important factor for the slope stability; the influence includes the intrinsic factor and the extrinsic factor. Generally speaking, the affect of intrinsic factor includes the gravity density and strength of the soil layer in the slope. Groundwater also may produce another additional force in slope soil, such as Buoyancy pressure, Hydrostatic Pressure, Hydrodynamic pressure, Penetration and so on, which are the extrinsic factor. Based on the study of every instability stage and every influence factor of a slightly-expansive soil slope, the conclusions are shown as below. The result is unsafe for the slope stability, if the analysis only includes the influences of groundwater on weight density of soil and shear strength. Especially, when the slope soil is a slightly-expansive soil, its strength will be reduced if it is wet. It should use the index of Water Immersion Test to check the stability of the slope, and the additional load should be considered, which is generated by the seep of groundwater.