We introduce a novel calibration optical path of the cryogenic radiometer, which can avoid the repeated dismounting measurement and eliminate the negative influence of Brewster window effect on calibration result. The novel calibration optical path is used to calibrate the absolute spectral responsivity of the standard transfer detector at 633 nm, the results of which are compared with the ones of the previous structure. It is shown that comparing the previous results to the structure optimization, the measurement uncertainty of laser power reduces by a factor of 2, the measurement uncertainty on the absolute spectral responsivity of the transfer detector decreases by 15%, and the consistence of the calibration on absolute spectral responsivity is 4.0×10 3. The experiment result proves that the novel calibration optical path of cryogenic radiometer can effectively reduce the calibration uncertainty against standard detector and improve the accuracy of calibration.