Encryption method for programmable logic controller (PLC)

The invention discloses an encryption method for a programmable logic controller (PLC). The method comprises the following steps of: issuing matched license certificates containing user identification code information to a PLC production line and a PLC user; exporting user identification codes from the license certificates by using the PLC production line, and writing the user identification codes into an electrically erasable programmable read-only memory (EEPROM) of the PLC; exporting the user identification code information from the license certificate of a user by using background programming software of the PCL during programming of the user; compiling the user identification code information into a PLC target code when the background programming software of the PLC compiles a user program to generate the PLC target code, and storing the PLC target code in a FLASH memory of the PLC; and comparing whether the user identification code of the EEPROM is matched with that of the PCL target code or not during