In the thermal infrared spectral region, the radiometric temperature of flat non-isothermal mixed pixel varies with the wavelength if we assumed that its emissivity can be represented by r-emissivity. The typical flat non-isothermal mixed pixel composed of soil and vegetation is taken as an example, analyzing the non-isothermal effects on the radiometric temperature of the pixel, and giving a possible method to present the variation of radiometric temperature change. In order to validate the method, simulated hyper spectral thermal infrared dataset is generated. The validation results show that the method can characterize the change of flat non-isothermal mixed pixel with wavelength well and improve the inversion precision of r-emissivity of flat non-isothermal mixed pixel.