Abstract: In high-filled soil-rock mixture slopes, underlying bedrock is often excavated into steps to enhance the mechanical properties of the bedrock-soil-rock mixture interface and slope stability. However, the mechanical behavior of the stepped interface is complex and influenced by multiple factors, with the effect of bedrock lithology remaining unclear. This study proposes a three-dimensional refined numerical method based on continuous-discrete coupling. Direct shear test models of stepped interfaces between soil-rock mixture and bedrock of different lithologies were established. A series of numerical shear tests were conducted to systematically analyze the influence of bedrock lithology on shear mechanical properties of the stepped interface. Results show that: As bedrock lithology strengthens, the absolute rotation angle of rock and the number of force chains within the shear box both decrease. Lithology change has no significant effect on fractures distribution and quantity. During numerical simulation, rigid walls can represent bedrock with better lithology, but are not recommended for poorer lithology. The shear strength and stability of the interface improve with increasing lithological strength, but this improvement becomes insignificant once the lithology reaches moderately weathered mudstone. For practical engineering, further excavation to higher strength bedrock beyond moderately weathered mudstone offers limited stability enhancement but increases costs significantly, and is therefore not recommended if the moderately weathered mudstone meets design strength requirements.