X-ray communication(Xcom) is a method which applying X-ray as carriers to transmit information in space. After the concept of Xcom was produced by Dr Keith, many institutions and scientists have conducted researches on Xcom and achieved fruitful results. However, most researches are focused on the key devices of Xcom, such as the X-ray emit source, the antenna and the detector, there is nearly no theoretical model for space X-ray communication. In this article, we focused on the power transmission process of space Xcom. To begin with, we analysised the signal channel and transmission model based on the additional gussian white noise(AWGN) model and free space attenuation, established the power transmission equation and link model. Then we analysised the error source of Xcom, established the noise model based on single photon detectors and also given the main noise sources. In other words, we can built mathematical relationship between the power of X-ray source, transmitting distance&speed and bit-error-ration (BER). Finally, we testified the signal photons of Micochannel Plate(MCP) detector output against various anode voltage and modulations. The results accord well with the theoretical analysis, and the power transmission equation and BER model can explain the Xcom process well too. Based on this transmission theory, we can calculate the emit X-ray power against various transmission distance and BER level respectively. As a result, three key parameters which judging a wireless optical communication system have been given as well as the theoretical model. Simulation results showed that with a constant photons flow, Pulse Position Modulation(PPM) has a better BER level than On-Off Keying(OOK) modulation. When the transmission distance is about 10 km, in order to achieve BER less than 10－6 level, the power consumption of X-ray is less than 1mW. According to the power transmission model, in order to improve the BER level of Xcom system, we need a lower dark current detector, an advanced modulation and more transmitting power. These theoretical and experiment results could provide foundations for optimizing the core parameters of XCOM system in our future works.