Abstract: Scatter is an important problem in high-energy flash X-ray radiography. The X-ray transport of flash radiograph about FTO (French test object) was simulated by means of Monte-Carlo method to investigate the influences and the main source of side scatter. The results show that without side cone, the front cone is the main source of side scatter in the system. For any pixel, the scattered exposure from the front cone is more than 50% of the total exposure. And the side scatter has a monotony distribution in the axis direction. With side cone the side scattered exposure is decreased, and the distribution becomes complicated with single-peak in the direction perpendicular to the axis. In addition, the increment of the distance between the recording plane and the center of the target may not le