The functioning of an X-ray imaging system is well characterized by its point spread function (PSF). The point spread function consists of prime point spread function and scatter point spread function. A point spread function model is developed for X-ray imaging system. Parameters in the model include part thickness, source to detector distance and geometric parameters. Based on the model, an analytical expression for the scatter fraction is derived and it is a function of part thickness and source to detector distance. Experiment result proves correctness of the model by comparing calculated scatter fraction and the one measured from experiment. The model gives an estimation of scatter and geometric blur for their removal and is an effective model for X-ray imaging.