A group of improving methods for the on-ground calibration of solar backscattered ultraviolet spectroradiometer (SBUS) has been put forward to increase its precision of on-board detection and data inversion. The full-waveband radiometric calibration in vacuum, which can attain correspondence match of condition between on-ground calibration and on-board detection, is the first item. A spectral irradiance responsibility comparing measurement setup is built and the spectral irradiance response of SBUS to a same source is measured in air and vacuum, respectively. The result shows a 0.8% relative deviation between air and vacuum in 250~300 nm and a wavelength dependent deviation, among which the maximum is a little more than 15%, in 300~400 nm. The individual uncertainty caused by calibration condition of SBUS in vacuum is 1.8% smaller than that in air. After theoretical analysis and confirmatory testing, it is found that the spectral reflectivity of Al+MgF2 coating of SBUS reflecting element would be different in air and in vacuum, which proves the necessity of SBUS calibration in vacuum condition.