A measurement system has been developed for organic and elemental carbonaceous based on the thermal-optical method. The factors are analyzed which interfered the split of the organic carbon from the elemental carbon. The laser intensity correction is used to compensate the laser intensity′s fluctuation induced by the temperature′s severe changes during the thermal analysis. The radiative transfer model of transmittance and reflectance through the filter is developed, and the measurement theories of the thermal-optical transmittance and reflectance are described. The influence of different optical correction methods to the organic carbon and elemental carbon measurement results are discussed.