The complex chemical reactions in the Si-Cl-H system with relation to modified Siemens process have been studied in this paper based on the thermodynamic data of related substance. The influence of the temperature, pressure and initial feed ratio (nSi / nSiHCl3) on the silicon yield have been studied. Furthermore, the diagram of Kinetic constant k as a function of temperature for the rate controlled reaction has also been fitted in the SiHCl3 hydrogen system. Finally 1425K, 1.5atm and the initial feed ratio of 15 is the best conditions. Under these conditions, the silicon yield is 34.815%.