Prism is one of the most key parts in the hyper spectral imager (HSI). Consequently，to set thermal control target and make thermal control design, the thermal spectral property of prism in the HSI was studied. The working principle of the HSI and the definition of its thermal spectral property were introduced. The working environment of prism and its thermal effect were analyzed; also the study contents and technical route of the prism’s thermal spectral property were discussed. The effects of different uniform temperature field on deflexion angle and angular dispersion of the prism in the HSI were deduced, and the changes in displacement of the spectra and the spectral bandwidth under different uniform temperature were obtained. For one instance, the thermal spectral property of the K9 prism and the fused silica prism were compared based on FEM and combined experiments, furthermore, its thermal control target was ascertained and a thermal spectral property test was carried out to validate the rationality of the thermal spectral property analysis. The results of analysis indicated that the changes in spectral bandwidth and spectrum resolution brought by thermal distortions can be ignored according to current fixing mode, and the displacement of the spectra is mainly determined by thermal coefficient of material refractive index; because of it’s the lower thermal coefficient of material refractive index, the displacement of the spectra of the K9 prism is smaller under the same temperature changes; the material deflexion changes (dn/dλ) of prism are not sensitive to the temperature, so the changes in spectral bandwidth caused by them are not obvious. And the results of test proved that the studied method of thermal spectral property is reasonable and essential, and the results are authentic and credible. So it can provide some guidance for setting thermal control target and optimizing thermal control design.