Abstract: The relaxation and engineering characteristic of rock mass after slope excavation are studied based on the monitoring data from the slope excavation in a hydropower station. The results indicate that rock mass of the slope will be relaxed along the slope face after sideslope excavation and that the thickness of relaxation zone can be measured by engineering investigation, survey of seismic wave velocity and sound wave. The mechanical strength of rock mass will decreases after relaxation. Shoring immediately after excavation is one of the effective methods of preventing rock mass from relaxation.