As the reference detector of absolute radiance calibration primary radiometer (ARCPR) for space remote sensing, total solar irradiance (TSI) cavity is investigated to ensure the measurement accuracy. TSI cavity adopts a sloping bottom cavity. The geometrical parameters and paint material of sloping bottom cavity are both optimised to improve the invariance of absorptance in the environment of ground and space. The optimisation of substitution method is proposed to measure the average absorptance by two-dimensional scan. The uniformity is evaluated by absorptance map. Experimental results illustrate that sloping bottom cavity has a super-high absorptance of 0.999928 (wavelength is 632 nm), and the relative measure uncertainty is approximated to 6 ppm. Meanwhile, compared with the absorptance map of conical cavity, the sloping bottom design improves the uniformity of absorptance. Therefore, the sloping bottom cavity can be adopted as the reference detector of ARCPR. Furthermore, the optimised substitution method is suitable for the investigation of the blackbody cavity with super-high absorptance.