Abstract: This paper discusses the modelling of synchronous networked control systems(NCSs)with long random network-induced delays,and proposes a general model for these NCSs.In the process of modelling,the concepts of the effective arrival instant and the equivalent arrival instant of signals are introduced.Through the equivalent treatment of the actual arrival instant of packets,we construct a model for the NCSs in which the packets might be lost or received disorderly,and present a general model of NCSs.Based on this general model,we design an optimal controller for synchronous NCSs with long random delays with the help of state prediction and dynamic programming.The effectiveness of the controller is validated with an example.