Abstract: Aiming at the onboard calibration challenges of spaceborne imaging spectrometers with a field of view (FOV) exceeding 64°, a full-aperture onboard calibration system for the HY-3A programmable medium resolution imaging spectrometer (PMRIS) was designed. A dual-motor-driven diffuse reflector was employed, introducing sunlight and built-in light emitting diode (LED) light sources in a time-sharing manner. By combining the monitoring of diffuse reflector attenuation with a spaceborne radiometer, a composite calibration scheme was formed. Firstly, the overall scheme of the onboard calibration system, the calibration institution, and the monitoring of diffuse reflector attenuation were designed. Secondly, the onboard calibration methods and procedures were introduced. Finally, on-orbit performance verification was conducted. The verification of on-orbit data for over six months indicates that the radiometric calibration stability of PMRIS full-field is generally better than 1%, and the stability of the spectral central wavelength is better than 0.12 nm. This system successfully achieves precise monitoring of the on-orbit instruments’ performance, providing quality assurance for high-precision ocean color remote sensing data and reference for calibration design of similar ultra-large FOV remote sensors.