Abstract: In view of the high reliability in jacking system of offshore jack-up rigs, the PLC redundancy design is implemented in "KAN TAN QI HAO". The reliability index and the calculation method of redundant design are given in this paper. The redundant system is based on SIEMENS S7-400H PLC, and the main network adopts fiber ring type, the CPU as well as the key components are redundant. After system testing, the jacking operation can continue in the case of power module CPU and main network failure. The redundancy design makes the reliability of the jacking system greatly improved, and can meet the requirements of the marine environment.