Abstract: Bremsstrahlung source with high energy electrons will produce a substantial excess of high energy photons, and these photons and electrons severely degrade the fidelity of system generated electromagnetic pulse(SGEMP) simulations. Thus, the development of composite thin converter were undertaken to achieve a high fidelity of X-ray source, using high Z material with small thickness to lower photon energy and low Z material to filter the transmission electrons. The influence of converter thickness on X-ray parameters was analyzed by using MCNP code. And a composite converter was designed for flash-Ⅱ accelerator based on the simulation result. Experimental result shows that the average energy of photons is 121 keV, the dose on 700 cm2 area is more than 40 rad(Si) and it is 170 rad(Si) on 500 cm2 area. The uniformity is less than 2∶1, and the ratio of photons to electrons is more than 103. The proportion of the number of photons whose energy is less than 120 keV in total photons is 69 percent, and the proportion of energy is 58 percent in the same case. The proportions of simulation result are respectively 70 percent and 65 percent, which are close to the experimental result. The X-ray source can be used for SGEMP simulations and the design method of converter can be used to lower photon energy for other X-ray sources.