Bremsstrahlung radiation can produce pulsed X-ray, and the energy spectra and conversion efficiencies of X-ray produced by such radiation are related to the electron beam energy, target material, and structure of this material. In this study, electron beam spectra were obtained from the voltage and current waveforms of diodes, and the particle transport Monte Carlo calculation model of a composite target was established. Using the resulting model, the transport laws of electrons and photons in different materials were simulated, and the influence of composite targets was studied. The results were found to be useful in the design of composite anode targets to achieve low energy, high fluence, high conversion efficiency, and low transmission electron energy. The results also revealed that an organic glass softened the electron spectrum and attenuated transmission electrons in the composite anode target. The X-ray energy spectrum produced by the same thickness of a moderated target was found to be much harder than that produced by a thin target with the same thickness. Decreasing the thickness of tantalum was deemed to be good for reducing the average photon energy, while increasing the thickness was found to be good for improving the energy conversion efficiency. When a thin target with 2 cm organic glass and 10 μm tantalum was chosen, the average energy of the X-ray was 133.22 keV, and the conversion efficiency of the photon energy was 0.055%.