Emissivity is an important parameter of radiation thermometry. Based on Planck’s law, in the temperature measurement by radiation , there is an underdetermined problem, N equation containing N+1 unknown number, a new calculation method is put forward with the application of the emissivity of slowly varying characteristics, in order to reduce the number of unknowns, simplify the calculation process, we conducted theoretical and experimental measures to verify the calculation method. The results showed that both theoretical thermal radiation spectrum and the experimental data of the inversion results of emissivity spectrum and temperature were consistent with material emissivity and its real temperature. When T=1 173 K, the maximum difference of the inversion temperature is 13 K, and the maximum absolute error of the emissivity is 0.05,the greater the emissivity of slowly vary, the shorter the wavelength interval , the higher the accuracy of calculation is. Such method can be applied to obtain temperature and emissivity based on multi-spectral data.