Multilayer dielectric diffraction grating is the most important optical element in spectral beam combining system. The surface distortion of grating under high power laser irradiation will change its diffraction characteristic, which will influence the beam quality and diffraction efficiency of the combining system. We establish an experimental setup of Twyman-Green interferometer to measure the interference fringes on the diffraction grating surface, under the high power laser irradiation. Based on the image processing and Zernike polynomial fitting method, the grating surface profile is reconstructed. With the high power laser heating the grating and another laser measuring the beam quality and diffraction efficiency degradation, the results show that the grating surface deformation distribution conforms to Gaussian model, when the irradiance power density is 3 kW/cm2, the maximum distortion is evaluated to 127 nm, which leads to the diffraction beam quality factor M2 degradation. The degradation is mainly due to the increase of angle divergence after diffraction. The change of diffraction efficiency is less than 1.8%, which is basically unchanged.