Microwave radiometer imager is used for detecting cloud images in satellite for weather forecasting. Its mechanical scanning?component part is a bias parabolic reflector antenna with a large diameter, rotating at a low speed. Vibration amplitude is an important parameter of the microwave radiometer imager, overvibration will directly affect the satellite attitude control. It is proven that the vibration is mainly caused by dynamic unbalance. An insitu experiment for low speed dynamic balance of microwave radiometer imager is done. The results show that low speed dynamic balancing insitu is an effective method to reduce vibration of the microwave radiometer imager.