Abstract: Poly-α-methylstyrene (PAMS) is one of the most important mandrel materials used in the fabrication of laser inertial confinement fusion (ICF) targets. The effects of the molecular weight on the degradation products and degradation temperature of PAMS were investigated using fast pyrolysis gas chromatography-mass spectrometry (Py-GC-MS), thermogravimetric analysis (TG/DTG) and isothermal degradation activation energy of PAMS was calculated by Arrhenius equations. The Py-GC-MS results show that the pyrolysis of PAMS was found to yield α-methylstyrene monomer exclusively.Moreover, it also suggested that the effect of molecular weight on the degradation products of PAMS can be neglected. The TG results indicate that the thermal degradation temperature decreases as the molecular weight of PAMS increases, and the main thermal degradation temperature ranges from 240 to 450 ℃.Finally, with the decrease of molecular weight and the increase of thermal degradation rate, the activation energy of thermal degradation of PAMS increases.