The spandrel groove excavation of the Xiluodu Hydropo wer Station forms a slope of 250 m high, whose stability is a complicated space problem. The system calculation and analysis on stress field, displacement field and failure zone indicate the high rock slope of spandrel groove and water inle t are stable if they are excavated in the designed slope ratio. The local stabil ity analysis on the rock slope indicates there exist unstable rockmass blocks in the rock slope of spandrel groove and water inlet which have different level th reats to the excavation and operation of the slope. The relevant steps have been put forward to solve the problems about the stability of the slopes of the span drel groove and water inlet.