The modulation transfer function (MTF) is an important indicator that describes the performance of an optical imaging system. To avoid the effect of atmosphere on the measurement readings obtained by the on-orbit MTF on a meteorological satellite, the calculation method considering the lunar limb is investigated. First, we select the moon image obtained by FY-2G/VISSR as the input for the Knife-edge method. Then, we separate the image into sub-images according to different pixels of the detectors. Finally, based on different sets of sub-images, we obtain different results, which can improve the comprehensiveness of the MTF results. The experiments indicate that the result calculated by the Moon image is better than the result from the Earth image, which are 0.320 1 and 0.216 4, respectively. The standard deviation of MTF results from continuous sub-image is 0.013 31, which indicates the stability of performance of the system.