In order to improve the application level of laser technology, it is necessary to convert a Gaussian beam to a flattop beam. The basic principles and design theory of aspheric Gaussian beam homogenizer was analyzed, and the function of super Gaussian beam was chosen as the mathematical and physical model of flattop beam. The mapping formula of the incident Gaussian beam and the output flattop beam w established. Then, the surface coefficient of high order even aspheric was given. Moreover, the optical system of Gaussian beam homogenizer was designed with optical design software ZEMAX. The incident beam diameter was 2 mm, the output beam diameter was 4 mm and the wavelength of the laser was 1 064 nm. The energy conversion efficiency was analyzed, and the flattened degree of flattop beam was defined according to the Bessel formula. The design results show that the multiplying power of collimated laser beam expander is 2, the energy conversion efficiency is 95.98% and the flattened degree is 96.6%.