The propagation properties of flat-top Gaussian beams in free space, single lens system and complex optical system are investigated by numerical method. It is found that when a flattened Gaussian beam propagates in free space, it will first present a hollow structure, then two side peaks form, at last the two side peaks will gradually merge into a single-peak construction with the propagation distance, and the Gaussian-like construction of single-peak can maintain for a long distance. While propagating ina single lens system, the flattened Gaussian beam presents strong focusing effect at the focal point, and after a certain distance, the beam will return to the flat-top distribution again. However, in the complex optical system composed of aperture and lens, the flat-top distribution of beams cannot recover after the focal point because of the restriction of the aperture, which indicates that the aperture should be avoided in the path of flat-top beam.