Abstract: The No.1 spillway tunnel outlet of Centianhe project has large excavation section, steep slope, strongly weathered surrounding rocks and high unsymmetrical rock stresses. After the excavation of the upper part of the tunnel, the construction had to be stopped since the heavy rainfall induced cracks were found in the tunnel sidewall and the outlet slope. More reinforcement and monitoring measures were designed to help restarting the construction quickly, and then to insure that the reservoir could store water on time. A three dimensional calculation mesh was built to accurately reflect construction progress. The monitoring results of initial stage were adopted to estimate the mechanical parameters of the surrounding rocks, which were then used to predict the subsequent deformation and stability of the slope and tunnel. The results showed that the deformation induced by later excavation could be controlled under the enhanced reinforcement measures and the subsequent construction could be safe. The measured deformation increment induced by the subsequent excavation of the lower half of the tunnel and the open channel foundation was used to conduct the back analysis. The calculated results agreed very well with the monitoring data. Then the stress of the lining was predicted. This study helped restarting the construction of the tunnel, and also benefited the water storage and flood discharge of the reservoir.