In recent years, with the rapid development of the national defense, industry, technology and other fields, whether it is for the power transmission systems or for the steel smelting and new high-tech industry, temperature measurement has been of important significance. Especially in the high temperature and accompanied by the demand for transient （less than lus）temperature measurement occasion, multi spectral radiation temperature measurement method has been widely used. Multi spectral thermometry is selected by the measured target multiple wavelengths radiation information, the mathematical model of emissivity and wavelength is supposed, finally the radiation temperature is obtained. At present, when the method is used to measure the temperature, the spectral emissivity is fixed with the assumption of the mathematical model, and the fixed model is unable to adapt to the target under different temperature conditions. Similarly, at different temperatures, how to calculate the final emissivity and radiation temperature has been no universal method. Based on the Planck’s law of black body radiation, this paper proposes a new idea that is based on the form invariance of the spectral emissivity function under different temperatures. According to the method, the emissivity model adapte to the dynamic change of the object according to the object under different temperature conditions. At the same time, it also puts forward a general method to calculate the final emissivity and radiation temperature. Through a lot of simulations and experiments, it is proved that the method proposed in this paper is more simple and practical than the existing spectral emissivity solution, which can effectively improve the accuracy of the calculation of spectral emissivity, so as to improve the accuracy of the radiation temperature measurement. At the same time, the method proposed in this paper has the characteristics of good practicability and wide application.