This letter proposes a high-precision cosecant square beamforming reflectarray using an improved hybrid Particle Swarm Optimization and Genetic Algorithm (PSO-GA) and low coupling square ring element. Firstly, a novel hybrid PSO-GA algorithm is carried out to optimize the phase distribution of the high-precision beamforming reflectarray. Then, an element is presented whose reflection phase is insensitive to different incident angles and reflection amplitude keep stable with variation of element size. By using above mentioned methods, a high-precision beamforming reflectarray is designed and analyzed, the experiment results show well-defined cosecant squared beams in the predefined direction are achieved in the frequency range from 13.1 to 14.3 GHz and have a low loss.