A community antenna system in a closed loop mode and a method thereof are provided, the system includes two groups of pre-weighting processing modules, beamforming modules and antenna arrays connected in turn. The method includes the following processes: performing the pre-weighting processing on a transmission signal respectively according to the channel information corresponding to each of two antenna arrays; determining beamforming weight vectors according to two groups of steering vectors corresponding to the two antenna arrays, and performing weighting processing on the two pre-weighting processed transmission signals respectively to form two directional beams directed to the receiving antenna of a target mobile terminal; and transmitting the two directional beams via two antenna arrays with orthogonal polarization modes respectively.