The cracks on the ground surface will form a complete crack system gradually with the increase of the slope deformation. In view of the rainfall infiltration process of this type of landslide, it is pointed out that the preferential flow penetration developed along the dominant channel formed along the surface crack leads to the deterioration process of landslide stability and promotes the development of deformation. This paper systematically analyzes the relationship between the preferential flow penetration, the fluctuation of groundwater level and the slope deformation. The Wangjiapo landslide is investigate to reveal the temporal-spatial deformation evolution of the dominant channel and the corresponding deformation characteristics. And then the starting and ending points of the irregular "step" shaped deformation acceleration interval on the cumulative displacement deformation curve is defined as the "acceleration point" and "deceleration point". Finally, the groundwater fluctuation rule in the process of landslide deformation is inverted and the related stability evolution rule is obtained by using monitored data of W3 monitoring points. It is found that the "acceleration points" and "deceleration points" of the deformation acceleration interval of landslide are consistent with the starting and ending points of the stability changes. The method for determining the deformation acceleration interval can be established, and then the displacement rate threshold is determined.