Abstract: The main powerhouse of Three Gorges underground hydropower station is with large span and thin cover, and it is rare internationally. This paper analysed the deformation monitoring data of main powerhouse wall rock of Three Gorges underground hydropower station under construction. The results showed that until the excavation end of layer Ⅵ, the maximumdeformation of the wall rock was 17.59 mm. At present, changes of displacement tend to be stable and within the allowed design range. So the wall rock of main powerhouse is stable and safe. The safety monitoring during construction stage played an important role in feeding back design and construction guiding.