With the reinforcement project of non-pressure diversion tunnel of a hydropower station in Guizhou as background, this research aims at the engineering diseases such as deformation and cracking of side wall of tunnel, detachment of rock blocks from tunnel vault and upheaving of tunnel floor caused by weak expansive mudstone. According to the geological characteristics of the study area, we analyzed the deformation of surrounding rock by means of tests and geological radar, and found that the swelling of rock triggered by long-term water immersion is the main cause of engineering diseases. Moreover, by simulating the stress and strain distribution of sidewall in 3-D model, we proposed a restoration scheme involving drainage and water insulation, supplemented by anchor reinforcement and partition demolition in severely deformed sidewall area. In addition, we put forward some requirements for construction techniques, such as the installation of check valve on sidewall and the demolition and reconstruction of sidewall. The restoration project has finally achieved good practical result.