Based on the theory of vibrational relaxation, the buffer gas mechanic model is developed. The mechanism of buffer gas and spectrum characteristics of optically pumped miniature cavity NH3 far-infrared laser are calculated by semi-classical density matrix theory and quantum mechanics theory. The effects of operating gas pressure and pumping power on spectrum characteristics are studied quantitatively. And the theoretical results are verified by the experiments. The results shows that the spectrum of far-infrared laser can be widened when certain buffer gas is added into the laser medium, and the output power of far-infrared laser increase too.