Typical intensity distribution of laser beam is Gaussian profile. To improve the energy efficiency of the laser systems,it is necessory to convert Gaussian profiles or quasi-Gaussian profiles into uniform profiles. Based on the energy conservation and constant optical path-length principle,the spatial beam shaping system with aspheric lenses is analyzed in detail and the phase distribution for the optics is derived. In addition,a quasi-flattop intensity distribution is realized from the quasi-Gaussian beam by phase only liquid crystal spatial light modulator. The phase distribution of the shaped beam is measured by a wavefront sensor.