Based on the upper part of a certain underground powerhouse in the center of “5.12” Wenchuan earthquake, damage diagnosis and safety evaluation of the powerhouse under ambient excitation is studied. According to the time-displa cement data of structure observed on the site, the modal parameters of structure are identified by stochastic subspace identification(SSI) and eigensystem realization algorithm (ERA). Results are compared by the different methods. Combined with the dynamic characteristic of undamaged and damaged cases, a non-destructive testing and safety evaluation method is put forward for large-scale hydraulic structures. The health condition of structure can be quantified by distance concept. The testing results have a good consistance with the results of measure by boring. The study has referenced value for non-destructive testing and safety evaluation of similar large-scale hydraulic structures.