Focal spot size is a key parameter for evaluating the resolving power of the accelerator. A reduction in the focal spot size can effectively improve the spatial resolution of the object. This work studies and designs collimator structures for spatial restriction, which help to reduce the geometry blur of imaging and thus obtain a smaller effective spot-size. The Monte Carlo method is applied to simulate the generation of the light source and the imaging process of the spatial restriction structures. The parameters of the light source with different collimator structures are analyzed, including the distribution and size of the effective focal spot, the angular distribution and the spectrum of the photons. Theoretical calculations show that an effective focal spot size with a sub-mm scale can be obtained by means of spatial restriction at the expense of a partial loss of the field-of-view and the exposure.