Abstract: The failure of stratified rock slopes often results in repeated sliding along the bedding planes, and determining the length of the initial rupture is crucial for the design of support structures. This study focuses on stratified rock slopes that contain weak interlayers and considers the strain-softening characteristics of these interlayers and the stress release caused by excavation. The formula for calculating the length of the initial rupture of stratified rock slopes is derived by establishing the stress equilibrium equation for the sliding body micro-segments. The study also conducts a sensitivity analysis of the critical factors and verifies the reliability of the calculation formula through practical engineering cases. The results demonstrate that the dip angle of the rock layer, residual friction angle, slope length, layer thickness, and peak cohesion are the main factors that affect the size of the initial rupture. The length of the initial rupture is positively correlated with the dip angle of the rock layer, residual friction angle, slope length, and layer thickness. In contrast, it is negatively correlated with peak cohesion. The study also shows that the influence of residual cohesion angle, peak friction angle, slip band modulus of elasticity, and critical shear displacement on the calculation results is relatively small. This method is suitable for the sliding-tension failure mode of stratified rock slopes with weak interlayers.