Via solving density matrix equations of the three level system, the medium gain Gs and output power density Is along the sample tube in a miniature optically pumped NH 3 molecular far infrared laser (mini NH 3 OPFIRL) were calculated by means of the iteration method. The calculation results showed that the laser tube had an active region and a saturation region. For the mini NH 3 OPFIRL with a definite tube length, in a certain range the rise of operating gas pressure mainly resulted in an increase of peak value of Gn and shortening of the active region. Considering the scattering loss and the self-absorption effect, the increase of peak value Gn was not important to the laser output intensity. But, the rise of pumping laser intensity Ip0 was advantageous to the homogeneous activation along the light axis of the medium, which enhanced the total output power. Meanwhile, the signal frequency detuning obviously affected the amplifying processes in the mini-NH3-OPFIRL.