Medium-resolution spectral imager (MERSI) obtains global image and radiation data from the second generation polar-orbiting meteorological satellite, FY-3A, launched by China. However, the on-orbit gain levels of imager in shortwave infrared bands occasionally jump due to the unpredictability of space environment and other factors. Such phenomena limit the quantitative application of the data. The space view is selected to identify gain jumps based on the analysis of MERSI observation targets, such as space view, visible onboard calibrator, and blackbody. Automatic identification, gain classification, and level normalization are proposed as methods. The daily identification and intraday precision detection methods are combined to obtain the time and space of gain jumps, and the classified statistical method contraposing the data of gain jumps is used to obtain the on-orbit gain levels. In addition, the onboard calibration source targets and Earth view images are used to verify the normalized effect. The 91 gain jumps of 1.64 μm and 18 gain jumps of 2.13 μm are accurately positioned during the lifetime. In addition, the eight on-orbit gain levels and the levels of all gain jumps are obtained. The results revealed that the normalized effect is ideal. Further, the most gain jumps occur during the Earth view scanning process and the gain jump time varied between the detectors. Because of the complexity of the Earth view targets, it is difficult to realize the normalization of the image of the jumping frame based on the physical methods. These results contribute to the improvement of FY-3A MERSI image quality and play an important role in reprocessing historical data.