Abstract: This paper presents the evaluation and correction of the on-orbit calibration biases of the Microwave Humidity Sounder II (MWHS-II) aboard Fengyun 3D (FY-3D) against the Suomi National Polar-orbiting Partnership (S-NPP) Advanced Technology Microwave Sounder (ATMS) using the double difference method. First, a microwave radiative transfer model (RTM) and a calibration bias correction method are developed. Then, the matching observations in the year of 2022 between the MWHS-II and the ATMS are collected over both sea surfaces and land surfaces. Next, The simulated brightness temperatures for both MWHS-II and ATMS channels were computed. Based on the matched observational and simulated values, the double differences (DD) and the theoretical observations for FY-3D MWHS-II were then calculated. Finally, the calibration biases in the MWHS-II channels are corrected. The results show that the RTM constructed in this work is valid and basically accurate. In June of 2022, the calibration biases in FY-3D MWHS-II channels 1, 10~15 are -1.93±5.13 K, -3.89±1.40 K, -2.80±1.00 K, -1.03±0.97 K, -0.42±1.11 K, 1.53±0.82 K, and -0.87±1.31 K, respectively. In the whole year of 2022, the calibration biases in the MWHS-II channels are generally stable: in the MWHS-II channels 1, 10~15, the maximum variations in two consecutive months are 1.42 K, 0.30 K, 0.31 K, 0.36 K, 0.43 K, 0.37 K, and 0.39 K, respectively, while the maximum variations in the year are 2.03 K, 0.39 K, 0.37 K, 0.38 K, 0.53 K, 0.60 K, and 0.63 K, respectively. The results indicate that the on-orbit calibration biases of FY-3D MWHS-II are corrected against S-NPP ATMS.