A multi-input multi-output (MIMO) antenna system includes a metal plate, a background scanning antenna, a plurality of working antennas, and an isolator module. The isolator module is mountable on the metal plate and includes a plurality of isolators that are in a ring configuration to form an isolated space therein, so that the background scanning antenna can be located in the isolated space and a plurality of working antennas are located outside of the isolated space, thereby ensuring good isolation of the background scanning antenna from the working antennas by the design of the isolator module.