In the volume holographic storage, improving the holographic diffraction performance from the perspective of optimizing the preparation process of holographic grating is simpler and more efficient than introducing new components to improve the storage material, and has certain applicability. Polymethyl methacrylate (PQ/PMMA) photopolymer doped with phenanthioquinone was prepared by thermoinduced polymerization. The diffraction properties and volume shrinkage of PQ/PMMA in off-axis holographic storage system were studied under different material thickness, light intensity and interference angle. The experimental results show that the thickness of the material is linearly related to the diffraction properties, and the best light intensity increases with the thickness. The maximum spatial frequency that PQ/PMMA is about 1285 lp/mm. The volume shrinkage law of holographic grating calculated by Bragg curve migration shows that the thickness and light intensity are negatively correlated with the shrinkage, but positively correlated with the spatial frequency. This study is of great significance for the application of PQ/PMMA in off-axis holographic data storage.