A uniform flat-topped beam has an important application in the laser processing,laser-driven nuclear fusion,and so on. The flattened Lorentz function is used as the output beam intensity distribution,and the aspheric lens group is used for laser beam shaping. By analyzing the effect of the flattened Lorentz function and full width at half maximum (FWHM) on the shaping effect,the least square method is used to fit the aspheric surface. Finally,the lens group is simulated by ZEMAX software. The results show that the selected order is 25,and the shaping effect is better when FWHM increases,and the ideal flattened beam output can be achieved in the central area.