Abstract: The paper used determinate and stochastic Petri net to establish a model of message transmission delay of Profibus-DP single master-station system and did simulation analysis in TimeNet software. It got relationship between message transmission delay and target-token cycle time and gave a method of setting target-token cycle time reasonably according to parameters of Profibus-DP in actual projects taking STEP7 software as an example,so as to make monitoring and control system get better real-time performance. The simulation result showed correctness of the model.