The measuring precision of terahertz atmospheric propagation coefficient has an important impact on engineering applications of terahertz technology. A High-Q Quasi-Optical Resonator System for measuring the atmospheric attenuation factor is introduced, which measures the absorption spectrum of water molecules under different relative humidity to obtain the corresponding quality factor of the environment, and then the corresponding atmospheric attenuation factor can be calculated. The difference between atmospheric attenuation factor of 0.4 THz electromagnetic wave and the results of Millimeter- wave Propagation Model(MPM) are compared and analyzed. The results show that,the measured values and the model values are quite different under low relative humidity; while the measured values fit well with the model values when the relative humidity is above 17.79%.