Abstract: Ertan Hydropower Project is located on Yalong River in Southwest China, Sichuan Province. It is the largest under ground hydropower station in Asia. Six Francis units are installed in the powerhouse, which is located in the rock mass of the left side bank of the river. At the site of underground powerhouse intact syenite and basalts with mainly three sets of joints and very high initial stress are predominant. Reasons of triggering collapse in busbar galleries and tail pipes and large deformation in powerhouse happened last year are analysed by different means. Results are compared with the observation data obtained insitu. Some new opinions on the stability of the underground complex are presented.