A THz generation via stimulated Raman adiabatic passage in three-level Λ system was demonstrated. The basic model and theory had been analyzed in three levels coherent with pump laser and THz generation. The application also had been mentioned in D2O molecules optically pumped by a TEA CO2 laser. It produced THz radiation at the wavelength of 385 μm. The power of the THz pulse as a function of the vapor pressure, and a maximum THz power had been obtained. It was displayed that this technology is a robustness method of THz source.