Six FIR laser lines produced by a pulsed TEA CO2 laser pumped NH3 system have been studied experimetally. The dependence of FIE laser output energy upon the pumping energy,the pressure of molecular gases and the absorbability to pumping energy have also been measured. It has been shown that,firstly, the optimum pressure corresponding to a maximum FIB laser output increases proportionally to the increment of pumping frequency offset, and secondly, the maximum FIB laser output is observed under the pressure at which the absorbability to pumping energy is just about 90% instead of 100%.