In mountainous engineering construction, the stability of highly weathered, fractured breccia high-steep slopes is directly related to both project safety and the safety of human life and property, necessitating reliable scientific analysis and evaluation to inform slope treatment decisions. Existing studies have predominantly focused on soil slopes or rock slopes with distinct structural planes, while comparative investigations on sliding surface selection and calculation methods for highly weathered, fractured breccia slopes without obvious structural planes remain insufficient. This gap often leads to a lack of methodological specificity in engineering practice, resulting in significant discrepancies between calculated results and actual conditions. Taking this type of slope as the research object, this study systematically compares the computational differences among six slice methods, including the Swedish slice method, the Morgenstern-Price method, and the simplified Bishop method, under both circular and polygonal sliding surfaces. For the first time, this study identifies the simplified Bishop method, the Janbu method, and the simplified Janbu method as the optimal calculation combination for such slopes. Furthermore, a "dual sliding surface cross-validation" method is proposed, effectively addressing the issue of large deviations in single sliding surface calculations and their inconsistency with actual conditions. Engineering case verification demonstrates that the calculation results obtained using the recommended method are more consistent with the actual safety state of the slope, thereby providing accurate and reliable technical support for slope treatment decision-making in similar projects.