Abstract: The alternating layered distribution of sand soil and clay soil body creates condition for the occurrence of gully cliff collapses in Yuanmou valley. On the basis of previous studies, the mechanical mechanism of gully cliff collapse under flow lateral erosion was analyzed by the simulation test method. The results showed that the flow shear stresses of different flows were within 1.71-8.55Pa, and they were much larger than the critical shear stress (0.13Pa) of started sand moving. The flow scouring caused the lateral recession of sand layer. The formula of lateral recession distance of sand layer was ΔB_s=20.131C_tΔt(τ-τ_c)e^{-1.3τ_c/γ_s+0.0047. The main factor of gully cliff instability was caused by the lateral recession of sand layer. By the simulation test, the overhang soil occurred the rotating collapse (F_sb=0.767<1 when the lateral erosion distance of sand layer was 0.312m. When the overhang soil body appeared the tension fractures of 5, 10, 15, and 20cm, those cracks development also accelerated the instability of overhang soil body.}