In order to optimize the measurement of the responsibility and angle parameters of the large-aperture laser detector, and to reduce the discrepancy generated from the central-peak weak-edge profile of gaussian beam, a large-aperture flat-top laser beam generator was designed based on aspheric lens shaping and high-magnification collimating systems. A comprehensive shaping technology based on intensity distribution, aspheric coefficients, and optical path difference was presented. The results show that the aspheric coefficient of a Kepler aspheric lenis less than 20 μm, effective width is 5 mm, and the energy uniformity of the flat-top output beam is greater than 92.8%. And a 40× high quality collimating system was designed to match the aspheric lens to realize a flat-top beam of 150 mm aperture in 2 m distance. The measurement results show that the energy uniformity of the 150 mm flat-top beam is greater than 94.2%. This research is helpful to the engineering design of laser measurement and calibration system.