As the FY-3A Medium Resolution Spectral Imager(MERSI) takes a cross-track scanning by 40 detectors in a thermal Infrared Window band,its radiometric calibration is more complicate than those of traditional single detectors. Therefore,this paper investigates a multi-detector radiometric calibration method for the MERSI. Firstly, the operational calibration algorithm for the IR band of MERSI based on onboard blackbody detector-by-detector is proposed to eliminate the radiometric response difference strips.Then,based on the radiance ratios of each detector to reference detector at different blackbody temperatures, this algorithm conducts the normalization processing for SRF-different bias (spectral normalization)to alleviate the image strips by radiometric mechanism. To verify the accuracy of MERSI calibration,the inter-calibration between MERSI and Moderate-resolution Imaging Spectroradiometer(MODIS) is performed by using Simultaneous Nadir Observation (SNO).Experiments show that the difference of their brightness temperatures is about 1.0 K,which may come from their different bandwidths.