The reactive polyurethane hot melt adhesive (HMPUR) was synthesized via bulk polymerization of poly(butyleneglycol adipate) (PBA), poly(ethylene orthophthalate) (PEO), poly(tetrahydrofurandiol) (PTMEG), 4,4-methylene diphenyl diisocyanate (MDI) and pentaerythritol diacrylate (PEDA). Its thermal degradation was studied by thermogravimetry (TG) and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). In helium atmosphere, the sample was pyrolyzed at three characteristic temperatures of 280, 450 and 600 degrees C according to the results of TG analysis. More than 20 characteristic volatile pyrolytic products were identified by on-line MS. The pyrolysis reaction types of HMPUR were proposed and verified through the analysis of pyrolytic products.