The integrated areas of UV visible absorption in 350～700 nm region have been used to characterize the thermal stability of PVC samples.The integrated absorbance of IR bands at 1667 cm -1 and 1650～1560 cm -1 have been used to measure the contents of isolated double bonds and conjugated polyene of PVC samples respectively. The results indicates that the coloration during the initial thermal degradation stage of PVC is dependent on the contents of conjugated polyene and the coloration during long-term thermal degradation is related to the conents of isolated double bonds.