The MicroWave Temperature Sounder (MWTS) on FY-3A has four-channels with designed band central frequencies of 50.3, 53.6, 54.9 and 57.3 GHz, respectively. The central frequency for three upper-level sounding channels is shifted after the satellite launch into orbit. This study investigates the characteristics of MWTS O-B associated with the frequency shift and demonstrates the utility of NWP models for diagnosing microwave instrument anomaly. The O-B is defined as the difference between observed brightness temperatures and simulated brightness temperature from NWP 6 hour forecast fields. Through tuning the central frequency of MWTS in the Community Radiative Transfer Model (CRTM), the MWTS O-B can be made similar to those from MetOp-A/NOAA-18 AMSU-A. It is shown that the adjusted FY-3A MWTS brightness temperatures after central frequency shift match quite well with the MetOp-A/NOAA-18 AMSU-A data at the simultaneous nadir overpass (SNO) locations in both the Arctic and Antarctic.