The fiber coupling modes will affect the fiber coupling efficiency and integrity of information, for this reason, the fiber coupling based on micro aperture diffraction has been studied from the direct coupling, micro circular aperture coupling, and micro lens coupling to micro distortion lens coupling. The changes of beam type, intensity distribution and spot size have been investigated, and then the fiber coupling based on fish eye lens is proposed to obtain the focused spot with a flat top. Computer simulations show that the coupling mode with fisheye lens is able to improve the coupling performance.