Different from visible spectral remote sensing, thermal infrared radiances vary with both temperature and emissivity. Hence, for thermal infrared remote sensing, the separation of temperature and emissivity is a key problem. In this study, by starting from WIEN’s approximation, ALPHA difference spectrum was defined and derived which is independent of temperature. Then, a correction item to remove the influence of WIEN’s approximation on ALPHA difference spectrum was developed. On these bases, a new temperature and emissivity separation algorithm was developed under the condition of neglecting the effect of atmosphere downwelling radiation, which combines attractive features of ALPHA derived emissivity method and ASTER TES algorithm. Compared with current TES algorithm, the new algorithm is fast, simple and precise. It can be used for wide temperature and surface type ranges.