Abstract: In order to study the initiation and formation mechanism of the failure mode of homogeneous soil slopes with double shear bands under different erosion conditions, an improved layered reduction principle was proposed in this paper. The comprehensive reduction factor (CRF) and attenuation hysteresis ratio (AHR) were introduced. The typical failure mode of double shear bands based on the layered reduction principle and the evolution law of shear strain were obtained. Afterwards the position and size of the shear strain peak at different AHR were analyzed, the threshold conditions for the attenuation spread ratio (ASR) when double bands emerge and stabilize were obtained. A comparison has been made between the ASR values at the intersection points and stable points of the cumulative release fraction (CRF) curves. The rationality of using CRF as the criterion for determining the emergence and stabilization of double bands were discussed. The research shows that the evolution of shear strain of typical double shear band could be divided into three stages, namely, accumulation towards slope, splitting and rapid expansion towards slope, and rapid expansion at the inner side of slope. With the increase of AHR, the peak shear strain moved to the slope and tends to be stable. In the moving stage, the peak value 1 decreased and the peak value 2 increased, while the total peak shear strain did not change. According to ASR threshold conditions, the slope instability process of different AHR could be divided into three sections single shear zone moving to the slope, double shear zone moving to the slope and double shear zone stable position and the rationality of CRF criterian condition was varified.