Stress gradient (angle deflection) of polymethyl methacrylate (PMMA) plate is measured by digital gradient sensing method(DGS). DGS first uses the digital image correlation method (DIC) to measure the full field displacements in the region of stress concentration of PMMA plate under the three-point bending. Then, angular deflection of light rays can be linked to stress gradients of PMMA plate according to the photo-elastic effect, and the angular deflections of PMMA plate can be further obtained. The angle deflections that are measured directly are compared with the analysis data, and errors are found. Because the in-plane motion of the rigid body and the out-of-plane motion exist unavoidably in the test, so the error of the angle deflections appears. After removing the above influencing factors by compensation regional method, we discuss the influence of the in-plane motion of the rigid body and the out-of-plane motion on the test. Finally compared with the analytic value, the results show that the compensation regional method reduces the measurement error, and the high-precision angular deflections are obtained. The average relative error of angular deflections in the x and y directions are 4.90% and 5.89%, respectively.