Abstract: The post-earthquake deformation mechanism of the 2015 Nepal Mw7.8 earthquake is studied by using the relatively perfect theory of spherical displacement and GPS data of one year after Nepal earthquake. Two different models of post-earthquake deformation mechanism are explored: ① Single post-earthquake afterslip model (model 1); ② Combined model of post-earthquake afterslip and viscous relaxation (model 2). The results of model 1 show that the post-earthquake afterslip mainly occurs at the depth of 20-35 km and is located in the downdip area of co-seismic rupture; the main component of afterslip is thrust, accompanied by dextral strike-slip components, of which the largest component is 20 cm and 11 cm, respectively; the moment released by the post-earthquake afterslip is 1.23×1020 Nm, equivalent to Mw7.33 earthquake. The residual slip distribution obtained by model 2 is consistent with model 1, but the cumulative slip is slightly smaller, and the released seismic moment is 1.1×1020 Nm, equivalent to Mw7.32 earthquake. Furthermore, the results of model 2 show that the optimum values of lithospheric elastic layer thickness and mantle viscous coefficient in Nepal seismic source area are 40 km and 2×1019 Pa·s, respectively. To sum up, the post-earthquake afterslip effect plays a dominant role and the viscous relaxation effect plays a secondary role in Nepal within one year after the earthquake.