Based on geological investigation, a numerical calculation model was established by FLAC  3D to simulate the collapse and supporting consolidation during the first-stage excavation of underground powerhouse of the Dagangshan hydropower station. The consolidation effect and the influence of subsequent excavation after collapse on the stability of collapsed area were studied. The results showed that apart from larger  deformation  in the crushed stone zone, the deformation increase in other crucial locations was within 3mm after the top arch was reinforced by cable and steel arch. The results indicated that the consolidation effect was quite good； and as the powerhouse was excavated, the rockmass stress around collapse zone tended to be even due to the stress adjustment and the interaction among cable, arch and surrounding rock； the tensile stress of collapse zone rock almost disappeared； the anchor force increased and extended deeper gradually, indicating that the anchor worked effectively.