The Total Solar Irradiance (TSI) cavity in an Absolute Radiance Calibration Primary Radiometer(ARCPR) for space remote sensing should have an absorptance more than 0.999 9 and the measurement uncertainty superior to 0.001%. Therefore, a cylindrical sloping bottom cavity was used as a receiver cavity for calibration of the TSI because of its superior performance in the space and cryogenic environments. Then,the absorptance of the cylindrical sloping bottom cavity was measured. The characteristics of the cylindrical sloping bottom cavity were introduced, and why the cavity was used as the receiver cavity in cryogenic radiometer was analyzed. Furthermore, the principle of the absorptance measured by a substitution method was expounded. A reference light path was added to monitor the stability of a laser to improve the measurement repeatability and accuracy. Finally, the absorptance of sloping bottom cavity was measured using this method, and the uncertainty of measurement results was analyzed. Experimental results indicate that the absorptance of sloping bottom cavity is 0.999 928±0.000 005, superior to that of the calibration standard of the blackbody cavity, which verifies it is feasibility to use the sloping bottom cavity as the TSI cavity for measuring the TSI. The experiments also verify that the ratio of single voltage and reference voltage could be used to calculate the cavity absorptance, and could improve the uncertainty of measurement result. This method is fit for measuring the absorptance of an ultra high absorptance cavity.