Based on the semi-classical theory，the pulsed-laser pumped D2O gas tera-Hz laser was theoretically analyzed，the relation between the intensity of pumping laser and the output tera-Hz laser light was studied and numerically calculated. According to the result of calculation，the intensity of the output tera-Hz laser light increased approximately linearly with that of the input laser in a certain range，but gradually decreased because of the bottleneck effect when the intensity of the input laser exceeded a fixed value，and for given gas pressure，cavity length and operating temperature，an optimal intensity of pumping laser exists. A frequency tuning range of the output signal occurred when the intensity of the input laser was strong enough.