This study reports a method for the calibration of onboard polarization based on sun glint. First, a rough-surface ocean model was used to analyze the polarization radiation characteristics of the sun glint from the ocean. For radiation transmitted through the atmosphere, the 6SV radiation transmission model was used to calculate the polarization characteristics at the top of the atmosphere (TOA). Further, to correct the effects of aerosols, a correction formula for TOA polarization was proposed; the t test was used to verify the significance of the new quadratic term of the correction formula. Finally, the correction coefficient lookup table for solar zenith angles was established to calculate the simulated degree of polarization. Using polarization data obtained from the radiance observed by POLDER 3, the relative error was verified to be less than 2%. Compared with the degree of polarization observed by the multi-angle polarization imager (MAPI), the polarization calibration accuracy was approximately 2%, which meets the requirements of the instrument polarization precision. The uncertainty showed that the synthetic uncertainty of polarization obtained using this method was 1.43%.