The semiclassical density matrix equation of four energy levels was solved. Then by means of iteration method, the output power increment ratio of V 2∶a→s P(5, 4) to a→s Q(5, 4) transition in miniature optically pumped NH 3 molecules far infrared superradiant laser (mini NH 3 OPFIRL), which was pumped by CO 2 10R(6) line, was calculated. The calculation showed that the mode competition between the V 2∶a→s P(5, 4) and V 2∶a→s Q(5, 4) in superradiant mini NH 3 OPFIRL led to the elimination of V2:a→s P(5，4) and V2:a→s S(5，4) in superradiant mini-NH3-OPFIRL led to the elimination of V2:a→s P(5，4) transition branch. The theoretical calculation was consistent with the experimental result.