In a space-borne differential optical absorption spectrometer, which has a large field in nadir push-broom mode the “Sun+Diffuser” method is adopted for onboard spectral calibration. Therefore the aluminium diffuser used in the space-borne spectral calibration system is required to have a good Lambert feature to ensure the full field spectral calibration accuracy of the space-borne differential optical absorption spectrometer. And it can provide a uniform source in the observing view-field of the instrument. Using bidirectional reflectance distribution function measurement instrument, bidirectional reflectance distribution function of aluminium diffuser is measured by the relative measurement method. Experimental results show that in a wavelength range of 180–880 nm and an observing view range from -70° to +70°, the bidirectional reflectance distribution function declines from middle to both sides and approximates the cosine distribution, showing that the aluminium diffuser has a good Lambert feature. The spectral calibration of the space-borne instrument is also presented with the system: high calibration accuracy is reached by the calibration system, with the maximum deviation being 0.022 nm, which meets the requirements for the accuracy better than 0.05 nm. The aluminium diffuser measured in laboratory can be chosen for the spectral calibration system.