Abstract: The prediction of landslide failure has always been a hot and difficult topic in the study of landslides. Water is the key factor for most landslides to slip under natural conditions，especially in the loess area.Because the water sensitivity of loess is very strong，it is known as “Nine out of ten landslides are caused by water”.The critical stress state of slip zone soil can be described by critical state line (CSL) directly when the landslide occurs. Therefore，it can provide a basis for the prediction of landslides through studying the quantitative relationship between water content and critical state and determining if the slip zone soil is destructed. For studying the prediction method of loess landslide，the critical state line of slip zone soil is applied. Firstly，taking slip zone soil of Zhengcheng landslide as subject，consolidated undrained triaxialshear test (CU test) under different water contents were conducted. The critical state lines of various moisture contents are obtained. Taking the shear strength parameters as bridge，the relationship between the moisture content and corresponding critical state line of soil has been deduced. The results show that：critical state lines under different water contentsare different obliquestraight lines related with shear strength parameters,and critical state of slip zone soil is determined by water through shear strength parameters. After the water content is 15.28%，the critical state line of slip zone soil with high moisture content is closer to the bottom; The cohesion c increases first and then decreases with the water content，and the maximum value is 134.72 kPa; But the internal friction angle φ decreases with the increase of water content，and the sensitivity of c value to water is higher. In the end，by using the exploration data and empirical data of Hexi landslide in Liulin County，the feasibility of the critical state line in the prediction of landslide instability is explained. It is also concluded that the proportion of slip zone soil reaching the critical state line when the landslide occurs is about 60%.Which can provide guidance for the occurrence of loess landslide.