In southwestern of China, the stability problem of the complex high slopes subjected to excavation has become one of the key technical problems in the hydropower project construction. Based on the geomorphic conditions, characteristics of geologic framework and stress measurement data of the left bank slope of Baihetan hydropower station, by using the discrete element analysis procedure UDEC, a calculation model was established to study the stability of the left bank slope during excavation unloading. The principal stress field, deformation field and plastic yielded zones distribution law were studied under the condition of excavation, then the deformation mechanism of the left bank bedding rock slope has been declared. Meanwhile, the potential damage zone and critical sliding surface in deep rock mass were identified and delineated, by combination with the microseismic activity at the left bank slope during the periods of excavation and stress redistribution. The comprehensive studies indicated that the deformation of the left bank slope is mainly affected by multiple factors, such as tectonic stress, geological structure and unloading. The weak structure planes, such as LS3318, LS3319, LS331, LS337 and F17, play a decisive role in the excavation deformation of the left bank slope. The research results can provide valuable references for excavation and reinforcement of similar bedding rock slope projects.