The spectrum not-matching of a calibration source with target source can affect star sensor measurement precision in the calibration of star sensor. Quantitatively analysis the different colour temperature light source due to the spectral mismatch to the focus light source at 0 magnitude. The results show that the star sensor calibration error caused by spectrum mismatch is up to 0.323 magnitude. Put forward a kind of spectrally tunable light source, which can simulate any spectral distribution of stars and applied to the star sensor calibration. The experimental results show that calibration accuracy can be enhanced within 0.62%.