Based on plasmonic metal grating mechanism and multilayer theory, a subwavelength slit with metal-dielectric multilayer composite surface grating which can enhance focusing effect is proposed. According to the finite difference-time domain (FDTD) method, the numerical simulation results demonstrate that the focusing behaviors of the designed structure is related to the numbers of metal-dielectric layers and grooves. It is found that the focal length can be increased significantly by controlling the number of metal-dielectric layers. The maximum focal length and the maximum focus depth can reach 9.15 and 26.38 \mm, respectively. Therefore, the focusing performances are enhanced obviously compared with conventional mono-metal gratings.