Abstract: It is pointed out that the depth of focus, diffraction from aperture and radiation scattering are three main sources for image blurring of CCD receiving system. The radiation scattering is studied by MCNP simulation. The distributions of energy deposition in CsI:Tl crystal under different energy spectrum and depth of screen are given. It is shown that energy deposition increases linearly with the depth of screen increasing. The blurring from radiation scattering is related with the spectrum of photons. Furthermore, the blurring caused by the hardened spectrum changing with the depth of screen is less than that by non-hardened spectrum. It can be seen that the radiation scattering is the main source for image blurring, and the blurring from radiation scattering is different from the Gaussia