Abstract: In order to improve the efficiency of coupling Cassegrain telescope image disk into multimode fiber, we put forward a new method of putting an axicon duct close to the import of multimode fiber. By using total internal reflection several times in axicon duct, the rays could rip into multimode fiber. To determine the initial structural parameters of axicon duct, we analyzed the maximum coupling angle of image spot of Cassegrain at 0 field of view(FOV) by employing geometrical optical theory and numerical method. These parameters were used as the initial conditions of simulation in Zemax. After simulation optimization and tolerance analysis, the conclusions show that when the fabrication tolerance is controlled in 0.03mm, the coupling efficiency of image spot at full FOV is at least 4 times as the direct coupling.