The earth orbit millimeter wave radiometer detects the temperature and humidity profiles by atmospheric absorption band，using cold air as low temperature calibration source in orbit. When the calibration mirror points to the cold space to obtain the cosmic background radiation brightness temperature, the sun or other star affects the brightness temperature of cold air observation. Since the brightness temperature is much higher than 2.73 K，once entering it will seriously affect the observation values of cold air. In order to make full use of the observation data of radiometer，the influence of the sun on cold space observation brightness temperature is analyzed. A new method for the cold calibration of millimeter wave radiometer in a stationary orbit is presented，which makes the maximum deviation of the brightness temperature being reduced to 0.04 K. Other influence factors are analyzed as well.