Abstract: The paper analyzed operating mechanism of data link layer of Profibus-DP protocol, described time parameters that compose Profibus-DP bus cycle in details, and gave concrete calculating formulas of real-time indexes of Profibus-DP system named message cycle time, token target rotation time and system reaction time. Combining an example of some Profibus-DP single master-station system, it analyzed factors which affect real-time performance of Profibus-DP single master-station system through calculating system reaction time, and finally proposed methods to improve real-time performance of Profibus-DP single master-station system, including improving communication speed and decreasing number of substations and changed data.