This article mainly formulates that rainfall seeping makes the volumetric water of unsaturated soil changing, and the water content relates closely to soil suction. That means soil suction changes with the stability of whole unsaturated soil slope, that is, the safety coefficient has some relationship with volumetric water. So the “dangerous” volumetric water condition cna be determined under which, the unsaturated soil slope also is danger. By monitoring the volumetric water condition changing of unsaturated soil slope, some information of unsaturated soil slope stability can be obtained.