Large-aperture gratings have significant applications in inertial confinement fusion, immersion lithography manufacturing and astronomical observation. Currently, it is challenging and expensive to manufacture sizable monolithic gratings. Therefore, tiled multiple small-aperture gratings are preferred. In this study, the impact of seam phase discontinuity on the modulation of the laser beam field was explored based on the measurement results of the Shenguang-II laser large-aperture multi-exposure-tiled grating. An innovative method for accurately calculating the phase jump of multi-exposure-tiled grating seams was proposed. An intensive electromagnetic field analysis was performed by applying rigorous coupled-wave analysis to a reasonably constructed micrometer-level periodic grating seam structure, and the phase jump appearing in millimeter-scale seams of large-aperture tiled gratings was obtained accurately.