Abstract: Vanillin is mainly synthesized by guaiacol-glyoxylic acid method. In the distillation and purification stage, crude vanillin needs to be operated continuously at high temperature for a long time. Thermal decomposition of vanillin can be easily caused by long time heat exposure, thus affecting the quality and yield of the product. The thermal stability and thermal decomposition mechanism of vanillin were systematically studied in this paper. The purity and thermal decomposition products of vanillin in the process of heating at 130-150 ℃ for 24 h, and were studied by high performance liquid chromatography (HPLC), X-ray powder diffraction (XRD), thermogravimetery and differential scanning calorimetery (TG-DSC), and gas chromatography-mass spectrometer (GC-MS). Results showed that crystal morphology transformation was not observed during the heating process. With the increase of heating temperature and heating time, the proportion of vanillin thermal decomposition increased. After heated at 150 ℃ for 24 h, the thermal decomposition ratio of vanillin reached 5%. After heated for 24 h, the content of vanillin acid increased obviously.