In order to reduce the nonlinear effects in optical fiber effectively and make energy output of the light beam much more uniformly, a flat-top beam-shaping device based on micro-structured optical fiber (MOF) has been designed in this paper. Output of flat-top fundamental mode is realized by introducing a low refractive index inner core in the refraction-guiding MOF. Based on this principle, output performance of flat-top MOF is simulated and alayzed with the Rsoft. The structure parameters of the flat-top MOF are obtained after optimization, and the diameter of the corresponding mode field is 10.37 μm and the defect degree is 0.16%. These results show that the flat-top fundamental mode with large mode area can be carried out in the designed MOF, and moreover, they also provide theoretical basis for the fabrication of the flat-top MOF.