Experiments using a planar metal disc flyer driven by explosives and a cylindrical chamber was designed to synthesize cubic silicon nitride with the mixtures of α-Si3N4 and copper powders as starting materials. The ratio of transformation from α-Si3N4 to γ-Si3N4 approached to 80% percent at 45 GPa pressures and 4000K temperatures. The purity of γ-Si3N4 reached 100% after the synthesized samples were treated with hydrofluoric acid at 440K for 9-10h. High pressure sintering was carried out with a DS6×800A link-type cubic anvil apparatus at a pressure of 5.7GPa and calculated temperature of 1370-1670K over the course of 15 minutes. The result showed that γ-Si3N4 was completely transformed into β-Si3N4 at 5.7GPa, 1420-1670k and was partly transformed into β-Si3N4 at 5.7 GPa, 1370k. Micro-analysis indicated that the typical microstructure of sintered Si3N4 was elongated β-Si3N4 rod crystals in disordered orientation, the highest relative density of the sintered samples was 99.06% and Vickers hardness of them was 21.15GPa.