Abstract: Aiming at on-orbit radiation calibration of spaceborne low-light-level remote sensor, an out-site radiation (LED) light source based on the theory of radiation flux equivalence is designed. The designed calibration light source uses twelve 670 nm pick-wavelength LED modules, with each LED module max power dissipation of 300 W, and the max output radiance over 5.2×10⁻² W·cm⁻²·sr⁻¹. Based on the calibrating area of equivalent radiation flux, it can be applied to the on-orbit calibration of low-light-level channels in the order of 10⁻⁹ W·cm⁻²·sr⁻¹. The test results show that the planar uniformity of the source is better than 99.7% at the exit port, the angular uniformity is better than 99.6% within ±30° to the exit normal, and the stability of source is better than 99.9% within 1 h. In-orbit test of the low-light-level channel of VIIRS was carried out at the radiation correction field of Dunhuang Chinese remote sensing satellite, on-orbit response results was 8.27×10⁻⁹ W·cm⁻²·sr⁻¹. (including moonlight contribution and atmospheric impact). The rationality of the design of the radiation value of the out-site low-light-level calibration light source proposed in this paper is proved.