Based on spontaneous parametric downconversion process, we propose a novel self-calibration radiometer scheme which can self-calibrate the degradation of its own response and ultimately monitor the fluctuation of a target radiation. Monitor results were independent of its degradation and not linked to the primary standard detector scale. The principle and feasibility of the proposed scheme were verified by observing bromine–tungsten lamp. A relative standard deviation of 0.39 % was obtained for stable bromine–tungsten lamp. Results show that the proposed scheme is advanced of its principle. The proposed scheme could make a significant breakthrough in the self-calibration issue on the space platform.