The invention relates to a PLC warm backup redundancy method and system. The method comprises that a main PLC outputs a first signal on the basis of a received external signal and a control instruction sent from a host computer, and a backup PLC generates a normal logic signal on the basis of the external signal and the control instruction; the backup PLC collects the first signal output by the main PLC, and determines the mode, including a normal work mode, a fault mode and an error output mode, of the main PLC according to the first signal and the normal logic signal; and when the backup PLC determines that the PLC is in the fault mode, the backup PLC take over work of the main PLC. On the basis that the reliability and safety of the PLC system are not reduced, the service life of the system is prolonged, the fault tolerance is improved, and faults can be identified, positioned and eliminated instantly.