Large-aperture gratings are core components for pulse compression in kilojoule petawatt laser systems. The wavefront or amplitude error originated from fabrication and assembly of these gratings can be transformed into near-field modulation during propagation of the laser pulse. In severe cases, near-field modulation would induce laser damage on gratings and downstream optics. In this study, a three-dimensional near-field propagation model is developed based on ray tracing and diffraction propagation theory, allowing to quantify the effect of each grating in the compressor independently. We investigate near-field propagation properties of mosaic grating based compressor in detail, the impacts of periodic wavefront error and mosaic gap error of mosaic grating on near-field modulation is analyzed and evaluated, with two measured wavefronts introduced for further analysis. This work offers theoretical insights for estimating the fabrication requirement of gratings and reducing the risk of laser damage.