A novel lens arrays optical system with continuously variable focus width from several microns to several millimeters for providing uniform irradiation is proposed, which is composed of two lens arrays and an aspherical lens, and the system converts a circular laser beam into a flat-top square focused spot, which realizes the transformation of beam-shape and the uniform distribution of the spatial intensity at the same time. Based on the adaxial matrix optics and scalar quantity diffraction integral theory, the principle of this system is analyzed, and optimum design of the system parameters and numerical calculations for this system are presented in detail. The simulated results show that the nonuniformity along the x and y direction are ηx=4.6%, ηy=5.3% respectively, the energy efficiency can be up to 95.3%, and by moving the target slightly backward from the focal plane, off-focus length Δz=2 mm is obained, which nearly satisfies the need. The theoretical analysis agrees with the results of the emulational experiment with Zemax optical design software.