In the laser projection displays technology, The method by vibrating the fiber can effectively suppress the speckle and reduce the light loss. Generalized van Cittert - Zernike theorem was used in the theoretical analysis of speckle reduction in the laser projection display, where the vibrating fiber was used as speckle reduction approach. The results show that the speckle contrast depends on number of optical fiber model and the projection system pupil size. For the same size of the image plane, more models of multimode fibers should be collected and pupil diameter of projection system should be enlarged in order to reduce speckle contrast. In the projection system, the comparison experiment using two different multimode fibers were carried out and the results confirmed this conclusion qualitatively. This result provides a guidance for choosing the fiber production program and designing the illumination mode.