Abstract: The paper presents the results of three-dimensional analysis of Ertan large hydroelectric underground powerhouse by boundary element method （BEM）. The Ertan underground powerhouse, located in a high stress area, includes three large chambers linked by many tunnels and passages. The in-situ stresses, of which the magnitudes are very high up to 26.42 MPa, were determined by field overcoring stress measurements.A technique of reducing computer time in BEM proposed by authors was used. Stresses and deformations around chambers, tunnels and passages, particularly joints between them, are illustrated and discussed for stability of Ertan underground powerhouse. The results of analysis presented here could also be of value to stability analysis of other underground construction projects. In addition, from this computation we conclude that three-dimensional BEM is much better than three-dimensional FEM in input data preparation, result accuracy and computer time.