Metrological technique for traceability of an absolute radiometer to a cryogenic radiometer is studied. With laser as the light source, a calibration experiment is implemented in the radiation power mode in which the light spot does not fully cover the receiving plane of the absolute radiometer. The trap detector traceable to the cryogenic radiometer is used as the etalon to calibrate the laser, and the value is transferred to the absolute radiometer by substitution. The conversion from laser power to irradiance measured by the absolute radiometer is achieved by calibrating the aperture area of the absolute radiometer. The calibration uncertainty of the absolute radiometer used in the experiment is evaluated in detail by analyzing the calibration of the trap detector, the spatial response uniformity at different positions of the conical cavity in the absolute radiometer, and the wavelength selectivity of the cavity. The calibration uncertainty obtained is 0.86% (coverage factor k=2).