Abstract: An opened microstrip circuit is described imploying a three ports electric network in this paper.The errors caused by omiting of electromagnetic radiations of opened microstrip in numerical simulating of microstrip circuits is eliminated successfully by making one circuit port of the radiation.No priori procedure is needed,and the S-parameters of the-three-ports circuit are calculated from a single run of FDTD,which leads to a significant reduction in CPU time.A good agreement is found between the example and the data published,which shows that the definition of three-ports network used to describe the opened microstrip circuit is reasonable and effective.The definition can be widely used in the analysis of unshielded transmission,electromagnetic compatibility,microstrip antenna and CAD of opened transmission line circuits.