Tiled-grating technology is a possible approach to improve the high power laser output energy. For the protection of high-power laser beam temporal and spatial beam quality, stitching error of grating angle must be less than 0.4 μrad, and the piston deviation is less than 20 nm. A detecting system is designed to measure the error of rotation angle between tiled-gratings to meet the high accuracy and high stability of the tiled-grating assembly. A phase-shift interferometer is used to measure the detecting beam which is coaxial with the main laser beam, and some interference patterns are gotten and analyzed to reconstruct the wave front. The feasibility of the system has been verified by a series of experiments and the accuracy of the system is 0.45 μrad. Tiled-grating measurement program with only the requirements of wavefront tilt error, which simplifies the optical interferometry and image analysis process, will help grating tiling technology applied in engineering.