Abstract: With FactSage thermodynamic software, the thermodynamic equilibrium yields figure and deposition diagram of methyltrichlorosilane (MTS), boron trichloride and hydrogen system at high temperatures (900~1100 ℃) and low pressures (2 kPa, 5 kPa, 12 kPa) were calculated. The effects of total pressure, deposition temperature and ratio of precursor on equilibrium yields were discussed. The results show that temperature, total pressure and ratio of precursor have little effect on yield of SiC, but significant effect on yield of B₄C. Ratio of precursor has an influence on yield of C, but other parameters have little effect. The changes of parameter also influence the yields of main vapors (BH_x, C_xH_y, SiCl_x etc) in a certain degree. The increase of partial pressure of diluent gas is beneficial to formation of B-rich phase, and the increase of partial pressure of MTS favors SiC formation.