Formation conditions of the condensed phases (Si3N4 and Si) in CVD process of SiCl4-NH3-H2 precursors have been investigated in detail with thermodynamic analyses by using the FactSage code and its embedded database (44 species being involved). The productions have been examined at different SiCl4/(SiCl4+NH3) ratios, H2/(SiCl4+NH3) ratios, temperatures and pressures. The results showed that the condensed phase composition was quite sensitive to the ratios and temperature whereas it was insensitive to pressure. The ideal conditions for the deposition of Si3N4 were listed as followed: the ratio of SiCl4/(SiCl4+NH3) and H2/(SiCl4+NH3) was 0.44 and in the scope of 100.6-105.2, respectively. Temperature ranged from 1200 to 1300 K with lower pressure. Si was formed in the H2/(SiCl4+NH3) ratio of 103~105 and SiCl4/(SiCl4+NH3) ratio of 0.44-1.0. The formation of single-phase Si3N4 or Si would be easily controlled by changing the ratios of SiCl4/(SiCl4+NH3) and H2/(ZrCl4+BCl3). SiHCl3, SiCl3 and SiH2Cl2 should be the crucial intermediates in the process of CVD Si3N4 and could participate in the competition in deposition process. The results in this work were instructive for further investigation on the experiments under different conditions.