A communication system using a random code as an encryption code is disclosed. A first terminal transfers a request to the second terminal for providing a random code (rKey). The random code is used to encrypt commands in the proceeding communication process instead of using a master key (mKey) so as to avoid that the master key (mKey) is captured. The safety in data transmission is promoted greatly. In practical use, the encryption way can be used to a door access system which includes a mobile phone, a card reader, a door access controller, and a server program (such as ACX server program). The communication system using a random code as an encryption code assures that the communications between these devices are highly safe.