Abstract: In high energy flash radiography, enhancing the direct-to-scattered ratio(DSR) in receiving plane is favorable to the derivation of effective information. Monte-Carlo method is used to simulate the radiography process of French test object(FTO). Energy spectrum distribution of direct and scattering fluxes is obtained within the receiving domain. The method to improve the DSR in the receiving domain by energy spectrum optimization is proposed according to the distribution property. It is found that energy spectrum can be optimized by inserting an attenuation screen. Owing to its low absorption coefficient, tantalum is adopted as the material to optimize the energy spectrum with concerning practical manufacture condition. The maximum thickness of the attenuation screen is derived according t