Among the reshaping systems converting a Gaussian beam to a flattop beam, the system composed of a plane-concave and a plane-convex lens, is a relatively simple structure. In actual production processing, the asphericity of an aspheric surface is directly relative to its manufacture difficulty. Based on theoretical analysis of the conversion from a Gaussian beam to a flattop beam, the influences of several characteristic parameters on the sectional curves of aspheric surface of lenses are calculated and analyzed. Then, according to the actual requirements of input expressions in ZEMAX, a better structure of aspheric lenses system is obtained under condition that compactness is not destroyed and the asphericity is reduced. At last, physical optics propagation through the aspheric system is analyzed and then the irradiance distribution is discussed.