The AG-80 resin is a new type of thermosetting matrix for advanced carbon/epoxy composites. Mass loss effect and the related outgassing are major concerns for its application in space. The changes in mass loss, outgassing and chemical structure under 120 keV proton exposure were investigated for the AG-80 epoxy resin. The variation in chemistry was characterised by X-ray photoelectron spectroscopy. Experimental results show that by increasing the proton fluence, the surface colour of specimens becomes darker. Mass loss ratios ascend remarkably until the fluence of approximately 6.3×1015 cm-2 and then tend to level off. The surface roughness of specimens exhibits an increasing trend followed by a decreasing trend as a function of proton fluence. Under the exposure, the C‒C, C‒H, C‒N and C‒O bonds are broken, a variety of molecule ions with smaller molecular weight are formed and carbon is enriched in the surface layer of the specimens. The changes in mass loss and surface roughness of the AG-80 epoxy resin could be attributed to the formation of the molecule ions and the enrichment of carbon content in the surface layer due to proton radiation.