The surrounding rock mass of the underground powerhouse of Fengning pumped storage power station, in its construction period, exhibits various characteristics including large deformation magnitude, significant time effect, and occurrence of deformation at any depth of rock mass. The mechanism of rock mass deformation is analyzed based on geological conditions and monitoring data to study the distribution of rock mass deformation and its time effect characteristic. Results reveal that the deformation mechanism is, on one hand, related to the degradation of rock mass quality, reduction of rock mass strength, and rock mass alternation, and on the other hand, related to the excavation unloading induced open and dislocation of discontinuities, such as joints, fractures and faults that are distributed in rock mass. The Burgers rheological model combining Mohr-Coulomb yield criterion considering tensile limit is adopted as a compound visco-elastoplatic model to conduct back analysis of mechanical parameters of surrounding rock masses. The obtained deformation magnitudes and their time curves are in good agreement with monitored data. According to numerical calculation results, as of the completion of the fifth layer of main powerhouse construction, the rock mass deformation and plastic zone are both large. As the plastic zone runs through the rock mass pillar between the main powerhouse and main transformer hall, the rock mass stability is considered not favorable. Thus, it is necessary to suspend the subsequent construction after the fifth layer construction of main powerhouse is finished. A systematic reinforcement of anchor support towards the whole underground powerhouse is also required.