A method and fake base station for detecting subscriber identity are disclosed. Method includes The method includes: capturing at least one wireless message containing an encrypted subscriber identity of a fifth generation mobile phone system transmitted from a mobile subscriber apparatus; transmitting a reject message of the fifth generation mobile phone system for the mobile subscriber apparatus; capturing at least one wireless message of the fourth generation mobile phone system transmitted from the mobile subscriber apparatus; detecting an unencrypted subscriber identity from the at least one captured wireless message of the fourth generation mobile phone system; and transmitting a disconnect message of the fourth generation mobile phone system for the mobile subscriber apparatus.