As for the pollution concentration of rainwater runoff on the expressway changed randomly and complicatedly, a system dynamics model for pollutant concentration evolution in rainwater runoff was built to study the influence of traffic flow characteristics, expressway characteristics, and rainfall event characteristics on the pollution concentration of rainfall runoff. The simulation software, Anylogic, was applied to conduct fitting analysis on sample data of 21 rainfall events on five expressways. Through the sensitivity analysis on the parameters of rainfall duration, runoff depth, rain capacity, etc. , the time-varying regularity of the pollution concentration of rainwater runoff in different rainfall events was clarified. The results shows that the system dynamics model proposed in this paper can simulate the changes of pollution concentration in the rainfall events accurately. In the certain rainfall event with high intensity and short duration, the pollution concentration of rainwater runoff on expressway is relatively high, which has great influence on the receiving water body along the expressway. There is a positive correlation between the rainfall runoff depth and pollution concentration, and the max release concentration of pollution emerges earlier in the high rainfall runoff than in the low rainfall runoff. Therefore, in the early stage of the rainfall event, the average pollution concentration of rainfall runoff can be significantly reduced if reasonable control methods are adopted.