A zooming optical system based on a simple lens array used for uniform irradiation by a large-aperture laser is studied; and it can be shown that the size of target spot may be flexibly adjusted by changing the separation between the lens array and the focusing lens. How the choice of optical parameters will affect the zooming ability is analyzed, and a practical system is designed. Propagation of laser beam through the optical system is simulated numerically; results show that the spots in different size are similar in intensity structure. Speckle separation, modulation contrast, rms nonuniformity, and concentration ratio of the spot are quantified, and their variations are analyzed when the target is placed in different positions relative to the focal plane of the focusing lens.