Abstract: In order to achieve high-level collimation and the complete conservation of the flyer boundary in high energy flash radiographic experiment, this paper proposes a new kind of design method of the collimator. The coupling transport process of photons and electrons, taking FTO as the object, is simulated by the means of Monte Carlo method to validate the characteristics of the improved collimator. And the deduction method of the optical path of the improved collimator is investigated. The result shows that the scatter exposure at the core region of the object is reduced by 99.8%. The dynamic range of the signal is reduced by 99.7%,which is great benefit for designing the detector. And the outer boundary is completely preserved in the image. The optical path of the object with relative error less than 0.1 is obtained by the way of layering the secondary collimator along the axial direction.