The modified Siemens process, which is the major process of producing polycrystalline silicon through current technologies, is a high temperature, slow, semi-batch process and the product is expensive primarily due to the large energy consumption. Therefore, the zinc reduction process, which can produce solar-grade silicon in a cost effective manner, should be redeveloped for these conditions. The SiCl2 generation ratio, which stands for the degree of the side reactions, can be decomposed to SiCl4 and ZnCl2 in gas phase zinc atmosphere in the exit where the temperature is very low. Therefore, the lower SiCl2 generation ratio is profitable with lower power consumption. Based on the thermodynamic data for the related pure substances, the relations of the SiCl2 generation ratio and pressure, temperature and the feed molar ratio (n(Zn/)(n)Sicl(4)) are investigated and the graphs thereof are plotted. And the diagrams of Kp T-T at standard atmosphere pressure have been plotted to account for the influence of temperature on the SiCl2 generation ratio. Furthermore, the diagram of Kp T-T at different pressures have also been plotted to give an interpretation of the influence of pressure on the SiCl2 generation ratio. The results show that SiCl2 generation ratio increases with increasing temperature, and the higher pressure and excess gas phase zinc can restrict SiCl2 generation ratio. Finally, suitable operational conditions in the practical process of polycrystalline silicon manufacture by gas phase zinc reduction of SiCl4 have been established with 1200 K, 0.2MPa and the feed molar ratio (nZn/(SiCl4)-Si-n ) of 4 at the entrance. Under these conditions, SiCl2 generation ratio is very low, which indicates that the side reactions can be restricted and the energy consumption is reasonable. (C) 2014 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.