In this study, a low Earth orbit (LEO) space environment simulation facility, capable of simulating major LEO space environment constituents such as high vacuum, ultraviolet (UV) radiation, thermal cycling and atomic oxygen (AO) atmosphere, is introduced. Under the simulated LEO space environment, graphite/epoxy composites, which are widely applied to space structures, were tested to study the LEO space environment effects on the composites. Tensile properties, as well as mass loss of the graphite/epoxy composites, after being exposed to AO atmosphere and the synergistic LEO space environment, were investigated. The surface morphology of the composites under LEO space environment effects was also observed by scanning electron microscope (SEM). Experimental results showed that LEO environment and its synergistic effects cause considerable damage to the surface of composites.