To ensure absolute calibration of the solar band of the Medium Resolution Spectral Imager (MERSI), a cross-calibration algorithm utilizing Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) instrument over six Pseudo-invariant Targets (PTs) is developed and used to calibrate FY-3C＼MERSI data. The technique involves three key steps: Capturing the actual Bi-directional Reflectance Distribution Function (BRDF) characteristics using the well-calibrated SeaWiFS radiance data; Parameterizing the Spectral Band Adjustment Factors (SBAF) between SeaWiFS and MERSI sensor; and Cross-calibrating each screened MERSI observation based on the above two modules. The employed BRDF model can accurately predict ρTOA as observed by SeaWiFS with small biases (within approximately 3% for most cases). Furthermore, the fitted SBAF are nearly unbiased estimations of the simulated values. The time series of the cross-calibration result is very stable and exhibits no obvious trend, which is consistent with the characteristic of long-term stability and regular change of radiation signal of PTs. Hence, there is a systematic deviation from the calibration result of L1. This method fully considers and solves the uncertainty due to orbit drift, i.e., the difference between observed geometric conditions and spectral response in cross-calibration, and can effectively monitor and correct the radiation performance of satellites in orbit.