In order to realize measuring the absorptivity of cryogenic radiometer absorbing cavity at the 4 K temperature, a method of measuring the absorptivity with variable temperature is researched. By designing reflection monitoring components in front of the Brewster window of the cryogenic radiometer, and controlling the cryogenic radiometer to work in a vacuum environment of 10^-6 Pa, then adjusting the refrigeration temperature of the cryogenic radiometer, the reflection signals of the cryogenic radiometer absorbing cavity are measured at 632.8 nm under room temperature and different temperature conditions. Combined with the measurement results of the reflectance at 632.8 nm of the cryogenic radiometer absorbing cavity at room temperature using the traditional integrating sphere method, the absorptivity of the cryogenic radiometer absorption cavity can accurately obtain under different temperature conditions through calculations. The experimental measuring the absorptivity of the absorbing cavity at room temperature and 4 K temperature, the absorptivity is 0.99976 and 0.999 71, respectively. The measurement uncertainty of the absorptivity of the cryogenic radiometer absorption cavity at the 4 K condition is evaluated, and the results obtained show that the relative expanded uncertainty is 0.005%(k=2).