The conjugate angles of plastic flow network in the lower lithosphere, including the lower crust and litho-spheric mantle, are changed from right angles as their original state to obtuse ones during compression at the plate boundary, and, therefore, the magnitudes of strains can be estimated from the increments of the conjugate angles and the strain field be given. The study of three plastic-flow network systems in central-eastern Asi-a indicates that the strain field in the lower lithosphere controls or influences the tectonic deformation in the upper crust and the topographic variation on the surface, showing the correspondence between strain "peaks" and topographic peaks, the low-strain effect of "stable" blocks on the strain field and the wave propagation of strains. The method of estimation of strain from conjugate angle of plastic-flow network provides a new approach to understanding the intraplate strain field and tectonic deformation.