With the networked development of control systems, control networks come into being. Control networks connect every component in networked control systems (NCS) and are the key of NCS, but they make the problems in control systems involving in control theory and communication networks. The real time of control networks determines the one of NCSs to some extent, so it is emphatically researched in this paper. Some wired and wireless control networks are discussed. Then, the data transmission procedure in control networks is analyzed, as well as the network-induced delay produced by exchanging data among the network nodes and the methods on how to improve the real time of control networks. Finally, a type of three-layer distributed database system is proposed, as well as a type of monitor & control layer real-time database structure, of which the design and implementation is discussed in detail. The distributed database system can optimize the data storage architecture of NCSs, lighten the network load and reduce the network congestion, so it has good effects on improving the real time of control networks.