Wireless communication device capable of adjusting at least one antenna to improve efficiency of other coexisting antenna(s) and related wireless communication method

A wireless communication device is used for performing wireless communication via at least one of a plurality of antennas. The antennas include a first antenna and a second antenna. The first antenna includes at least one first controllable component. The wireless communication device has at least one communication system and a control circuit. The at least one communication system is used to perform the wireless communication via at least one of the plurality of antennas. The control circuit is used to set the at least one first controllable component according to a first setting when the first antenna and the second antenna are active, and set the at least one first controllable component according to a second setting when the first antenna is inactive and the second antenna is active, where the second setting is different from the first setting.