In order to convert a laser Gaussian beam to a collimated flat-top beam, a new reshaper with only one aspheric lens is designed based on double aspheric lenses reshaper. In the theoretical model, a more accurate calculation method of curves function of aspheric lens is given. The reshaping effection, brought by the change from the parameter of system and laser beam, is given through numerical analysis and optical design software simulation. It is illustrated that the flatten degree of the laser beam can be improved apparently using this new kind of reshaper. The novel method avoids some shortcomings of traditional refractive optical reshaper such as large volume, complexity of regulating, etc, and enhances the reliability and practicability of device.