The 2021 7.4 Maduo earthquake within the Bayan Har block allows us to understand the lithospheric mechanical properties in northeastern Tibet. We analyze possible postseismic processes by modeling the six‐month postseismic deformation from Interferometric Synthetic Aperture Radar data. Our optimal afterslip‐poroviscoelastic model is not only physically more reasonable but also has the best data fit. The postseismic deformation is dominated by afterslip located at depths of 10–20 km, revealing a complementary pattern with the up‐dip high‐coseismic slip. Yet ignoring poroelasticity and viscoelasticity will result in errors in afterslip of >12 cm locally. Poroelastic rebound contributes to significant postseismic line of sight (LoS) deformation, up to ∼14 mm, near the northwest and southeast fault segments. The steady‐state viscosities in the lower crust and upper mantle are and , respectively, consistent with the “jelly sandwich” model but not with the crustal‐flow models featuring much lower viscosity.