Alkali-metal vapor cell is one of the most important core component. As a sensitive device, it is often applied in the measuring device of the ultra-high sensitive inertial and magnetic field, which is based on the atomic spin exchange relaxation free regime. The content of the gas in the alkali-metal vapor cell will have an enormous influence on the relaxation of the atoms and the selection of other system parameters. Therefore, the precise measurement of each gas pressure is of great significance. Pressure broadening and frequency shift will occur when there is gas in the alkali-metal vapor cell. We merely took pressure broadening into account, because pressure broadening was much larger than the natural broadening and Doppler broadening. Since both pressure broadening and frequency shift had a function relationship with the gas pressure, we proposed a method and experimental equipment to measure gas pressure based on saturated absorption spectrum. Firstly, we obtained the optical depth curve by scanning the absorption spectrum of alkali metal atoms, and fitted the curve by Lorenz function. We were able to obtain the superimposed pressure broadening and frequency shift of single alkali metal atom, which was caused by mixed gases in the cell. Then, we solved the simultaneous equations to get the pressure of each gas, based on the known pressure broadening and frequency shift of single alkali metal atom, which were caused by a single type and specific pressure gas. The maximum number of gas pressure we can measure was 4n in the mixed gas, when there was n kinds of alkali metal atoms. Simulation results showed that the method was applicable when the incident laser was not completely absorbed. The effect on measurement accuracy caused by laser power and frequency fluctuations is insignificant. Temperature fluctuation is very significant. We should take measures to achieve the accurate control of temperature.