For the past few years, optical interconnects technology has been developed rapidly. At the same time, many problems and difficulties need to be solved immediately. There is a difficult problem in optical coupling that the coupling efficiency between fiber and slab waveguide is quite low. The coupling efficiency impacts the property of optical devices directly. The purpose of the paper is to design a kind of waveguide coupler, which is symmetrical in the structure, to achieve the practicable coupling between fiber and slab waveguide. And this kind of structure uses the subwavelength constraint characteristics of Surface Plasmon Polariton to make light break the diffraction limit of light, which is also helpful to optical transmission in slab waveguide. The result of counting by COMSOL™ and FDTD solution shows: on the communication wavelength λ=1550nm, the coupling efficiency between fiber and slab waveguide can reach around 50%.