Abstract: Aiming at the plane instability problem of horizontal-strata landslide which is regarded as a system and based on the establishment of mechanics model,the potential function model is applied to derive the mechanics failure criterion of instability of landslide system and critical deformation displacement of sliding soil theoretically.According to the evolution system of horizontal-strata landslide,the critical displacement is 34mm according to the derived formula;and then the relationships between the gap to critical catastrophe status and external environment factors which include the water depth in trailing edge fracture of landslide and water content of sliding soil is proposed,i.e.when the water depth in trailing edge fracture of landslide and water content of sliding soil are larger,the gap to critical catastrophe status is smaller,and the sliding is easier.