The methods for improving the focusing properties of subwavelength metallic lens by optimizing the groove width and depth of the structure simultaneously are presented. We investigate how the variations of the width and depth of the groove affect the half width at half maximum of the focus, intensity, normal focusing efficiency of the focal spot and the focal length, based on the finite-difference and time-domain method. The optimum design methods of metal lens are put forth through discussing the focusing regularity and physical mechanism, which can provide a theoretical basis for realizing the nano-focusing and flexible manipulation of light wave based on metallic lens.