Abstract: In order to use non-diffracting beams to extend the depth-of-field of optical imaging systems,the variation rules of the central spot of non-diffracting beams were investigated.Based on the theory of Fresnel diffraction,the diffraction patterns were analyzed when the point source was placed in front of focal plane,at focal plane and behind focal plane.Then the radius and intensity calculation formula of the central spot of non-diffracting beams correspond to the distance from the point source to the lens or the distance between the partial coherent point source and the lens,were deduced according to the theory of partial coherent light.Finally,experiments were done by using a red high brightness LED as the quasi-monochromatic light source.The experimental results showed that the radius of the central spot decreased with the increase of the distance from the point source to the lens,and the intensity of the central spot increased with the increasing of the distance from the point source to the lens.The outcome provides stated academic gist to the applications of non-diffracting beams in imaging system.