By means of the iteration method, the output power density of the miniature optically pumped NH3 submillimeter wave (SMMW) laser was calculated based on the density matrix equations of a quantum system. Optimization of operating parameters including operating gas pressure, reflection coefficients of input and output meshes of the laser were studied systematically. In the paper, the concepts of the average activated length and the synthetical optimum value were defined, and some rules of the optimization were explained successfully. Experimentally, a cavity NH3 SMMW laser that was composed of a pair of inductive metallic meshes and pumped by TEA-CO2 10R(8) line was used. The experimental results were in good agreement with our theoretical calculations.