Pumping heavy water gas (D2 O) molecules using continuous operation carbon dioxide laser 9R(22) output line, continuous terahertz laser radiation output with 385 μm wavelength and 0.78 THz frequency can be produced. The laser process can be effectively analyzed with the semi-classical theory analysis method combining three level system model, and the solving process was simplified through reasonable approximation. The analytical expressions of terahertz laser signal gain coefficient Gs and pump light signal absorption coefficient Gp were obtained, and the pump threshold for producing terahertz laser oscillation was calculated to be 6.24×10－3 W/mm2. Normally, when the pump laser output spot area is 0.25 mm2, the output power of its 9R(22) spectral line needs to be above 1.6 mW for stimulating terahertz laser radiation.