Large-size reflective grating is the core element for improving both the resolution of astronomical spectrometers and the output energy of chirp-pulse amplification systems. With the development of astronomy and laser fusion, the fabrication of large-size reflective gratings has become a hot topic for scholars both domestically and internationally. Compared to the fabrication of a single large-size reflective grating, the mosaic method has become the primary method of fabricating large-size reflective gratings, as it has the advantages of a low difficulty coefficient, low production cost, easy fabrication, and high quality of small gratings to be mosaicked. In this study, the basic principles of large-size reflective grating mosaic technology are introduced, and the research progress of grating mosaic technology is summarized in detail. More specifically, this study considers the theory of grating mosaic error detection, the separation of grating mosaic error, the phase correction of the wavefront of mosaic grating, the reduction of the dimension of grating mosaic error, and the grating mosaic device. Finally, the advantages and disadvantages of grating mosaic technology and directions for future development are summarized.