X-ray radiography has more and more application value and X-ray simulation system has practical significance for improving the quality of X-ray imaging and replacing the expensive devices but it’s a time-consuming work. This paper proposes an improved fast ray-tracing method extended from Siddon’s and Zhao’s methods. For a 2D array, we compute level by level which has natural modes, by comparing the incident x-coordinate with the characteristic constant. In the case of 3D models, the volume data are back projected into 1D linked list and compute indices and lengths similarly like the case of 2D. Compared with Zhao’s method, the new method avoids skipping parametric planes and computing several starting voxels and ending voxels in one level. The time consumed in the new algorithm has reduced by 5/6 regarding the conventional Siddon’s algorithm.