Because of the high altitude of Qinghai, its air pressure is 0.77 atm which is lower than the atmospheric pressure. Therefore, an analysis is done for carbochlorination reaction of flyash for the extraction of aluminum in such an environment. The phase diagrams and predominance area diagram on chlorides are calculated and drawn and the equilibrium compositions of complex system composed of multi-components are simulated by Factsage software. Results show that, carbochlorination reaction converts Al2O3 and SiO2 which can not directly react with chlorine gas originally to chloride completely, and this demonstrates the thermodynamics feasibility of carbochlorination process. The ratio of FeCl3(Fe2Cl6) to FeCl2 changes with the amount of carbon and chlorine gas. The catalysts like Na2CO3, CaCO3 and B2O3 have some side effects in the aspect of thermodynamics such as consuming chlorine gas and carbon. The volatility of AlCl3(Al2Cl6) is affected by other chlorides. Compared with the normal pressure region, there are advantages and disadvantages to carbochlorination reaction under the pressure below the normal. The work provides thermodynamic data for the process.