For the problems of huge structure, complex controlling way and uneasy manipulation and maintenance of control system, a control system network architecture based on three-layer network and four-tier structure is proposed for the drilling rig of offshore platform. For the requirements of high reliability, availability and security to the drilling rig, a design scheme for the monitoring system of dynamic fault tolerant redundancy based on Siemens S7-400H is put forward. The network of the monitoring system at different layers is designed redundantly. Based on the information channel structure, an information channel structure model is established to analyze the reliability of the monitoring system. The analysis result shows that the system is of higher reliability.