A general Monte Carlo program has been developed to study the performance of the X-ray intensifying screens and imaging systems.The program contains three major parts, each of which describes one of the interaction steps occuring in the system:the absorption and scattering of X-rays, the diffusion of generated electrons, and the diffusion of light photons.Through comprehensive verification, it shows that the simulation methods for all physical processes are correct and the results obtained are credible.For a kind of Gd₂O₂S:Tb intensifying screen, several characteristic parameters such as the number of collected light photons, light collection efficiency and PSF depending on the incident X-ray energy are calculated.These basic physical quantities are useful for the prediction of the information transfer properties of a phosphor layer.