The model of buffer gas effect on optically pumped submillimeter wave laser was established. And the deactivation rate of buffer gas was deduced by quantum theory, so that the buffer gas effect could be calculated quantificationally. The CO2-9(R30) laser pumping NH3 67.2um line was calculated theoretically and measured experimentally. The experiment results agree with the theoretical calculation. This work would be in favor of finding high-efficient buffer gas of optically pumped submillimeter wave laser.