Based on the theory of vibrational relaxation,the buffer gas mechanic model was developed.The mechanism and effects of buffer gas on the energy exchange and output power of optically pumped far-infrared (FIR) laser were calculated and studied by semi-classical density matrix theory and quantum mechanics theory.Laser output intensity was calculated and discussed in case of different operating parameters such as operating gas pressure,and the optimization rules were concluded.Finally the theoretical results are verified by the experiments.The results showed that:the vibrational relaxation time could be decreased and the efficiency of energy exchange could be increased when certain buffer gas was added into the laser medium,thus the output power of NH_3 FIR laser could be greater.And there existed an optimum ratio of gases mixture and an optimum operating gas pressure,which could make the output power of FIR laser reach maximum.