Based on the semiclassical density matrix theory and vibrational relaxation theory, the present paper studies the influences on optical pumping THz lasers output power due to adding an appropriate proportion of buffer gas and appropriately reducing the waveguide core diameter. Results prove that adding appropriate proportion of buffer gas or appropriately reducing the waveguide core diameter can increase the output of light intensity of THz laser. Optimizing the two parameters at the same time can further improve the efficiency of pumping laser energy into THz laser energy, extend the effective activation area of the working cavity, put off the appearance of the pumping saturation effect, and increase the output power of the THz lasers. This research may have a guiding significance for the THz optical pumping laser in improving energy conversion efficiency, and increasing the output power so as to bring about the miniaturization of THz optical pumping laser.