Abstract: With the rapid development of the control technology, bringing network into traditional control system and realizing long-distance close-loop control has gained more and more attention. But negative effect of the network-induced delay caused by information collision and resending can not be ignored. This paper proposes two methods about delay estimation and the delay data procurement on-line to deal with the random delay of networked control system(NCS), then sets up a mathematical plant model. It makes use of z-transform to dispose delay, generates residual through parity relations, decouples disturbance by parameter design, then diagnoses the control fault of the networked system, and provides the simulation results.