We re-estimated the seismic moment of the 2011 M_w 9.0 Tohoku earthquake by using far-field coseismic displacements from 41 continuous GPS stations in Northeast Asia. In support of nearly 100 GPS campaign stations, we interpolate precise interseismic velocities at continuous GPS stations which are lacked of pre-seismic observations by using the Gaussian distance-based interpolation method, aiming to pursuiting more accurate post-seismic displacements. Then, we invert for the mantle viscosity and effective lithosphere thickness in the study area by employing nonlinear least squares fitting of the high signal-to-noise horizontal post-seismic displacements from 2.5 to 4.5 yr after the earthquake at 9 GPS stations distributed perpendicular to the fault plane of the earthquake, and the viscoelastic spherical Earth dislocation theory with modified fault slip model. The optimum solution of the inversion realizes the viscosity being 1.0× 10¹⁹ Pa s and the lithospheric thickness being 30 km. We find that the far-field post-seismic displacements caused by afterslip and mantle viscoelastic relaxation are approximately equal within 2.5 yr, and after 4.5 yr the cumulative effects of the latter component dominate the total far-field post-displacements. The result indicated that we cannot ignore the effects of mantle viscoelastic relaxation in the far field, even immediately after this earthquake. In addition, in the study of the far-field post-seismic displacements, we should take the signal-to-noise ratio of the GPS data into account with adequate observations to ensure the accuracy of the inversion.