Abstract: X-ray photoelectron spectroscopy (XPS) is one of the important methods for qualitative and semi-quantitative analysis of surface elements of materials, especially the chemical states of the elements. By connecting mass spectrometry (MS) within its analysis cavity, surface element changes and gas products can be obtained simultaneously, thus realized the in situ analysis. Based on the obvious decomposition phenomenon of explosive CL-20 under light irradiation, the changes of surface elements and gas products of CL-20 under electron beam irradiation were obtained based on XPS-MS. The experimental results showed that with the increase of irradiation time, the peak intensity of N and O decreased rapidly, while the peak of gaseous products can be obtained effectively by mass spectrometry. The result proved that XPS-MS analysis technology can effectively realize the in situ tracking of solid state electron beam induced degradation reaction.