With the combination of injection molding and gas pressure sintering, ceramic shaped parts with small size and high precision can be prepared in large quantities at low cost. In this paper, low density polyethylene (LDPE) and ethylene-vinyl acetate copolymer (EVA) were used as binders to prepare silicon nitride green bodies under injection temperature of 165 ℃ and injection pressure of 85 MPa. The complete silicon nitride injection molding process route was obtained by thermal degreasing process and sintering kinetics test. The effect of feedstock solid content on green body density, sintering density and Vickers hardness and the non-Newtonian index change of feedstock at 140~160 ℃ were studied. The results show that the optimum solid content of feedstock is 52.42% (volume fraction). The density of silicon nitride injection green body prepared under this condition is 2.10 g/cm3, the sintering density is 3.23 g/cm3, and the Vickers hardness is (15.24±0.34) GPa. The non-Newtonian index of feedstock is smallest at 160 ℃, that is, the rheological of feedstock is the best at this temperature.