In the present work, high-quality MgSiN2 powders were prepared by a combustion synthesis method, based on Mg-Si3N4-N-2 system. The effects of additive content (NH4Cl or NH4F) and N-2 pressure on phase composition and crystal morphology of products were investigated. The results suggested that both NH4Cl and NH4F additives alleviated agglomeration and decreased the grain size of MgSiN2. In addition, NH4Cl did not result in the formation of an extra impurity, while MgF2 residue was found when NH4F was introduced. Therefore, NH4Cl additive was considered as a better choice for the preparation of MgSiN2 powders in comparison with NH4F additive. According to the observation of some MgSiN2 whiskers and notably increased Si content under low N-2 pressure, the pressure of N-2 over 0.5 MPa was essential to have a complete reaction in the present work. Finally, MgSiN2-Si3N4 composite powders with quasi-spherical morphology were prepared by adding excessive Si3N4 and NH4Cl additive in reactant. The formation and growth mechanism of MgSiN2 grains was reasonably speculated based on the experimental results. Because of the remarkable merits, the as-prepared Si3N4-MgSiN2 composite powders showed great potential for alternative sintering aid in the sintering of Si3N4-based ceramics.