Abstract: Effects of operating environment thermal radiation interference on atmospheric brightness temperature measurement with ground-based K-band microwave radiometer especially for channels near 28.0 GHz and 30.0 GHz are studied. A model for simulating antenna temperature which expresses the energy received by the radiometer based on radiative transfer is derived and used to calculate the response of the brightness temperature measurements to parameters such as antenna specifications, radome, surrounding temperature and emissivity. Results show that the equivalent main beam efficiency (η_e) defined by 3 dB points is only 73.17% for a typical antenna, of which the half-beam half width α=3.1° and the gain G=30 dB. The value of η_e would be even smaller if factors like aperture radiation effect, shape-error, and occlusion and so on are taken into account. The brightness temperature would fluctuate by 4.0 K in case that η_e=70%, the surrounding temperature and emissivity would change by ΔT_g=10 K and Δε=0.05 around T_g=280 K and ε=0.85 if the radome can be neglected. The fluctuation would increase up to 9.6 K if the size of the opening in the radome is just for the main beam. Therefore, if the equivalent main beam efficiency determined by the antenna gain and 3-dB beam width for the current radiometer system is not large enough, variation of the operating environment must be taken into account during the correction of K-band brightness temperature measurement even though LN calibration of the radiometer system can be performed as manual-required. For this, a brightness temperature correction method for operating environment variation is suggested according to the theoretical relationship and the result from application to observations. Over one year application after LN calibration shows that the fitness and correlation between the observed brightness temperature after correction and the calculated brightness temperature with radiative transfer equation is obviously better than before, especially for channels of 28 GHz and 30 GHz.