A method and related device are used for a DVD (Digital Versatile Disk) system to authenticate the DVD system by generating an authentication code corresponding to an inquiring code. The inquiring code has a first portion and a second portion both having at least one bit. The method comprises: generating a first sub-authentication code according to only the first portion of the inquiring code, generating a second sub-authentication code according to only the second portion of the inquiring code, and combining the first sub-authentication code and the second sub-authentication code to form the authentication code.