A system for locking an electronic device is provided. The system includes an electronic device, and electronic keys electrically connected to the electronic device. The electronic key includes a plurality of resistors which are connected in series, the resistance value of the electronic key is changeable by coupling different resistors of the electronic key. The electronic device comprises of a processing unit and a function key. When the function key produces locking command in response to user operation, the processing unit obtains the resistance value of the electronic key and produce encryption key according to the obtained resistance value to unlock the electronic device. When the function key produces unlocking command, the processing unit decodes the encryption key and unlocks the electronic device when determining the obtained value matches the decoded encryption key.