Establishing the theoretical relationship between the unique properties of X-ray photon (large energy, non-dispersive, strong penetrability, etc.) and the link performance is the primary problem needed to be resolved in X-ray communication system design. This paper firstly outlines the research histories of X-ray communication at home and abroad, the effects of link parameters such as the detector sensitivity, the divergence angle and the propagation distance on the propagation loss are studied by numerical simulation. Then, the theoretical link equation of X-ray communication is derived, and the solutions to the key technologies which restricting X-ray communication performance are proposed in the end to provide theoretical support for X-ray application in large capacity and high reliable information transmission in space.