This project is located on the gentle slope of the third-level terrace in the upper reaches of the Dadu River. Because of the deep ice water accumulation layer and the inclusion of a large number of pebbles and silt lenses, studying the failure mechanism of the slope of the deep heterogeneous pebble accumulation body is critical. The cohesive force and internal friction angle are nonsynchronously reduced through the software program by comparing the slope equivalent to the plastic strain area calculated by nonsynchronous reduction step conditions with the stress and strain data monitored on site. Simultaneously, the displacement and load data of the sample during the staged loading process, were calculated in detail. The results proved that during the predetermined load application process, the pebbles are gradually shifted, and the silt lens interspersed between the pebbles is quickly destroyed. The friction angle between the pebbles provides the shear strength, further verifying the rationality of the nonsynchronous dual-strength reduction method. Compared with the general synchronous reduction method, analyzing the slope stability is more reasonable. Simultaneously, the double strength reduction method provides a solution to calculate the slope stability, and can be used for similar slope projects in the future.