The influences of the main parameters on population inversion including initial population of the ground state, statistical weights ratio of the lower and upper laser levels, ratio of the three body recombination cascade speed and the spontaneons radiative decay speed are calculated and studied by using numerical calculations. The practical system is analyzed and discussed, with an example the recombination pumping B like N 45.21 nm 3s 2p system based OFI (Optical Field Ionization). The results including the optimal laser intensity to realize B like N 45.21 nm 3s 2p laser amplification are presented.