Abstract: Edge detection of flash X-ray radiography image is a principal means to extract quantitative boundary information of primary implosion material. The precision of edge detection is an important index for judging flash X-ray radiography diagnostic level .In terms of feature of high-energy flash X-ray radiography image, typical Sobel edge detection algorithm in digital image processing is analyzed in this paper. A simple algorithm for threshold alternative and localization criteria based on maximum gradient is proposed to overcome typical Sobel operater’s disadvantages of the obtained image edges being thick and causing edge point misjudgement. The simulated result shows that the obtained edges of the image are thin and the edge position extracted by the algorithm is accurate.