Abstract: Channels in K band which frequency is between 20~30 GHz are chosen to retrieve atmospheric water vapor density profiles.Critical techniques and specifications of ground-based microwave radiometer are analyzed.Best combination of channel frequency and number selection is achieved and all channels are calibrated through nonlinear algorithm.Furthermore,sky-tipping method in the form of odd and even period is presented to test and validate the accuracy of nonlinear calibration procedure.According to radiosonde profiles,the retrieval profile after calibration has a root mean square error of 0.52 g/m³ with correlation coefficient of 0.965.