A multi-band multi-antenna system has become an important trend in the development of mobile communication systems. However, strong mutual coupling tends to occur between antenna elements with a small space, distorting array antennas’ performance. Therefore, in the multiple-input multiple-output (MIMO) antenna system, high isolation based on miniaturization of the antenna array has been pursued. We study in depth the methods of decoupling between antenna elements. Reasons for the existence of mutual coupling and advantages of mutual coupling reduction are analyzed. Then the decoupling methods proposed in recent works are compared and analyzed. Finally, we propose a metasurface consisting of double-layer short wires, which can be applied to improve the port isolation of antennas arranged along the H-plane and E-plane. Results show that the proposed metasurface has good decoupling effect on a closely placed antenna array.