Based on a two-dimensional viscoelastic spherical Earth model, this study constrains the mantle viscosity structure beneath the Sumatra region using GRACE RL06 satellite data and the VISCO2.5D code. The aim is to reveal the spatiotemporal characteristics of co- and post-seismic gravity changes induced by the 2004 Sumatra M_W9.3 earthquake and to analyze the lateral variation information of mantle viscosity contained within the gravity signals. GRACE satellite gravity observations were processed through filtering and smoothing to isolate the co- and post-seismic gravity change signals associated with the 2004 event. The extracted signal exhibits a negative-positive-negative pattern from southwest to northeast (Indian ocean-Sumatra-Malaysia). During the observation period from 2005 to 2012, the magnitude of post-seismic gravity change was comparable to the co-seismic change, with a maximum negative change of -3.7 μGal and a maximum positive change of 8.3 μGal. Considering the influence of the subducting slab near the trench, the observed large-scale co-and post-seismic gravity changes were simulated using the 2D viscoelastic Earth model and the spectral-element method in VISCO2.5D. The results show that when the mantle viscosity coefficients are set to 2×10¹⁹ Pa·s on the oceanic side and 1×10¹⁹ Pa·s on the continental side of the seismogenic fault, with a subducting slab near the trench and an elastic lithospheric thickness of 60 km, the simulated post-seismic gravity changes closely match the GRACE observations. This indicates that the post-seismic gravity changes in the Sumatra region are primarily driven by mantle viscoelastic relaxation, with a negligible contribution from aftershock effects.