The study of fifteen possible chemical reaction in the SiCl4 hydrogenation system in Siemens process based on the thermodynamic data for related pure compositions are reported in this paper. Curves of Delta G(m)(theta)-T for the 15 chemical reactions in the system have been fitted. The five independent reactions in the process have also been obtained and the relations of K-p(theta) and temperature have been fitted. Furthermore, diagrams of the equilibrium gas phase composition versus temperature under pressures of 1 atm, 3 atm, and 5 atm and feed mole ratios (x(H2) / x(SiCl4)) of 2 : 1, 3 : 1, and 4 : 1 are present herein. Finally, the influencing factors (such as temperature, pressure and the feeding molar ratio) on the conversion ratio from SiCl4 to SiHCl3 have been studied. Diagrams of the eta-conversion ratio are presented as a function of temperature, pressure and feed mole ratio, respectively. The optimum operation parameters in the actual production are determined by thermodynamic and kinetic studies. The results show that the conditions of 1100 degrees C, 3 atm and feed mole ratio (x(H2) / x(SiCl4)) of 4 : 1 are most preferable. Under these conditions, the conversion ratio from SiCl4 to SiHCl3 is 24.81%, contrary to the less than 20% in actual production.