The surface of silicon nitride (Si3N4) powder was functionally modified by hydroxylation combined with silane coupling agent (KH560) to prepare Si3N4 paste with high solid content and high curing depth. High strength Si3N4 complex structures based on stereolithography (SL) process were fabricated. The results show that KH560 on the surface of Si3N4 improves the compatibility of powder and resin, and reduces the viscosity of Si3N4 paste. At the same time, the epoxy group (—CH(O)CH2) of KH560 is combined with epoxy resin (EA) through chemical bonds or other ways to form an EA core-shell structure, which reduces the refractive index difference between the resin and ceramic particles. Thus, the curing depth of Si3N4 paste is improved. After the surface hydroxylation treatment, more KH560 is adsorbed on the surface of Si3N4, which further reduces the viscosity of Si3N4 paste and increases the curing depth of Si3N4 paste. Finally, the Si3N4 paste prepared with hydroxylated and KH560 modified Si3N4 powder has a solid phase content of 50% (volume fraction) and a curing depth of 64 μm. After sintering, the density of Si3N4 sample is 83%, the fracture toughness is (4.38±0.45) MPa·m1/2, and the flexural strength reaches (407.95±10.50) MPa.