The remote sensor mounted on the Fengyun-3C (FY-3C) will decay naturally during the operation, causing a decrease in the accuracy of the satellite data. Therefore, an effective monitoring method must be adopted to study the performance changes of the instrument. The theoretical expressions of the absolute and relative errors for the earth field of view (FOV) radiance together with its equivalent brightness temperature are given by analyzing the response value offsets of the earth FOV along with the blackbody (BB) pixel. In order to further describe the influences of different parameters on the absolute and relative errors for the FOV radiance and equivalent brightness temperature, the computer simulations are performed, and the absolute errors for the FOV radiance and equivalent brightness temperature caused the response value offset of the BB pixel firstly decrease and then increase with the increasing of the earth FOV response value. The absolute and relative errors for the FOV radiance caused the response value offset of the earth FOV firstly decrease and then increase, and the absolute error for the equivalent brightness temperature increases while its relative error decreases with the increasing of the response value of the earth FOV. This research provides an important theoretical guidance for improving the on-orbit performance of the FY-3C/ MERSI.