This study is concerned with the deformation field of the reinforced soil based on the consolidated drained triaxial test, using digital image processing technique in deformation measuring of soil specimen in trialxial test. In order to research the relation between anti-deforming capacity and strain of reinforced soil, intensity characteristic and failure mode, the glass fiber was used as a material of reinforce and 30 groups of triaxial tests were performed under 2 different reinforced positions and 3 types of confining stress. Together with digital image processing technique, we researched the transverse and vertical deformation ratios of reinforced soil in deformation process, drawn the deformation diagram of soil specimen in peaked strain and probed the reinforce mechanization and formation of shear field in reinforced soil. It was shown from the result that the geogrid has a great restraint, which increases with the growth of confining stress, on transverse deformation in medium sand. However, in final, the geogrid is incapable of changing the breakdown trend of soil body.