Abstract: Electrons energy distribution of FlashⅡ accelerator was calculated using current and voltage. The Monte Carlo Nparticle transport code was used for the radiation simulation, and the influence of target thickness and structure on X-ray parameters was simulated. The numerical results indicate that the average energy of photons increases with tantalum target thickness, and the conversion efficiency of X-ray has a maximum value. The ratio of total energy of photons to forward electrons distinctly decreases when polythene materials are added behind tantalum target. As an example, for 20 μm tantalum and 3 mm polythene targets, the average energy of photons is 102.68 keV, X-ray energy is 88.62 J which is greater than 0.02 J of the forward electrons energy, and the conversion efficiency is 0.57%. One composite target was designed for Flash-Ⅱ accelerator based on the simulation results and engineering conditions, and the simulation results agree well with the measured data.