Abstract: A new design method based on hybrid particle swarm optimization algorithm is proposed for low side-lobe beam-scanning reflectarray antenna. In this method,we defined a weighted fitness function firstly by the actual antenna pattern and the gradual ideal pattern,then searched the minimum value of this function by optimizing the aperture phase distribution,and achieve the purpose of lower sidelobe level ultimately. Meanwhile, a circular polarized beam-scanning reflectarray antenna with a center frequency of 12GHz was designed and fabricated. The measured results agree well with the simulated ones, the side lobe level is below 15dB within the scan range of 56°, the band can reach 11. 67%.