The directional polarimetric camera (DPC) on the GaoFen-5 satellite is not equipped with an on-board calibrator, and thus it requires different alternative methods for the in-flight radiometric calibration. For this purpose, an in-flight cross radiometric calibration method is developed, and the real-time surface directional reflectance and spectral response differences are considered to realize the in-flight cross radiometric calibration of DPC/GaoFen-5 and MODIS(moderate-resolution imaging spectroradiometer)/Aqua. In this method, MODIS/Aqua with a high accuracy on-board calibration ability is selected as the reference sensor, and the observation data over the pseudo-invariant calibration site in North Africa is used. By inputting the real-time satellite bidirectional reflection product and calculating the atmospheric radiative transfer, this paper accurately corrects the apparent radiation deviation caused by the sun-observation geometry and spectral response differences between the two sensors and finally obtains the in-flight cross radiometric calibration coefficient of the sensor to be calibrated. Theoretical analysis results show that the calibration uncertainty of this method is 2.41%-3.70% at the non-absorption band of DPC. The preliminary results of the cross radiometric calibration between DPC/GaoFen-5 and MODIS/Aqua show that they have a good radiometric consistency, and the apparent radiation difference of each band is less than 4%.