Colorimetry is commonly used to measure the surface temperature as true one when the surface emissivity is constant. That is, the target is regarded as greybody. But in fact, most materials are not greybody. Spectrum emissivity has to do with wavelength and temperature. And in the most case, it is difficult to assure the exact values of the emissivity .This makes the relative error of the temperature big. Therefore, it is necessary to modify the emissivity response of radiation pyrometer. The factor influencing the spectrum was researched through the application of least square method to create the relationship of spectrum emissivity with wavelength and temperature, at last mathematic model was established which provided numerical correction of infrared two-color thermometry.