Abstract: In the process of the excavation of underground powerhouse caverns at the right bank of Baihetan Hydropower Station, which is the world largest underground powerhouse in term of span, the surrounding rock at the roof arch from section 0 + 076 m to 0+133 m shows a continuous time-dependent deformation 17~26 m in depth from the excavation surface. The total deformation magnitude is 35 to 55 mm, and the deformation increments of 0~17 m in depth are synchronous. The special deformation becomes the most concerned rock mechanics problem of feedback analysis during the construction. On the basis of summarizing the monitoring layout and the results of arch displacement of the powerhouse, the spatial distribution characteristics of surrounding rock deformation are intuitively demonstrated by GoCAD interpolation of the monitoring results of multi-point displacement meter, and the particularity deformation mode of roof arch is explained. Then, base on FLAC^3D numerical analysis, the distribution characteristics of stress concentration, fracture expansion and time-dependent deformation of the surrounding rock of the cavern groups are discussed, and the formation mechanism of deep deformation is explained. Moreover, the failure phenomenon of the surrounding rock and the displacement monitoring of dense quasi-distributed grating fiber are used to verify the deformation mechanism. Finally, the stability of the surrounding rock of the roof arch with the phenomenon of deep deformation is evaluated. The research shows that the stress concentration level of the surrounding rock at the floor of the anchored tunnel above the roof arch of the powerhouse is greater than the crack initiation strength of the rock, and the fracture extension of the shallow rock mass causes the time-dependent deformation of the surrounding rock. Thus, the end of the installation base of multi-point displacement meter embedded in the excavation damage zone at the floor of the anchored tunnel is lifted, which indirectly results in synchronous and constant displacement increment at different depth measurement points of the multi-point displacement meter, thus showing the so-called deep deformation phenomenon. The research results may provide a reasonable explanation for the deformation monitoring mechanism of the surrounding rock of the roof arch and valuable technical support for the stability evaluation of the surrounding rock.