Abstract: In this article, a non-laminar and non-collision numerical simulation model based on a third order trajectory equation is presented. In this model, the injection angles of beamlets are assumed as a random distribution. Simulation result shows that X-ray spot size is almost certainly convergent when the number of beamlets increases gradually, and the convergent size is about 1.03 mm. The calculated uncertainty is in range of ±0.05 mm with different numbers of beamlets. In the experiment, X-ray spot size being 1.3 mm and focusing length 23.2 mm are obtained, which indicates that the maximum absolute error of the X-ray spot size is 0.4 mm and that of focusing length is 3.5 cm.