Abstract: EtherCAT is a type of real-time industrial Ethernet protocol for data communication in automation control system. Redundancy is an important way for hot swap and improving the communication reliability. The architecture, principle and frame structure of the EtherCAT were introduced. The fault tolerance and hot swap technology in EtherCAT based industry automation control system were proposed with ring topology. The master carries two full duplex Ethernet interfaces, forming two logical data stream. If there is a failure in the link, the EtherCAT slave will automatically loop back the data frames, so the ring topology is fault tolerant for one point failure. Redundancy frame transmission mechanism, fault location and recovering for network interface failure, link broken and node failure were laid out. The EtherCAT master driver with link redundancy was developed by adding the redundancy control to the normal EtherCAT master driver. It proves that the system is robust and has excellent performance of redundancy and hot swap function.