In order to clarify the influence of the main line traffic flow on the driver's mental load in the entrance and exit section under the high-density interchange driving environment. The natural driving real vehicle experiment was carried out in the high density interchange group of inner ring expressway in Chongqing. The Physiolab physiological detector was used to continuously collect the driver's ECG data during driving, which was used to count the changes of the driver's heart rate index. In this experiment, the heart rate variability indices RMSSD (root mean square of successive differences), LFnorm (low frequency norm), HFnorm (high frequency norm), LF (low frequency)/HF (high frequency) and Heart rate mutation rate index HRMR (heat ratemutation rate) were selected as evaluation indicators for driver mental workload using principal component analysis, denoted as principal component F₁ and principal component F₂. The results show that when the driver is driving in the entrance and exit section of the small clear distance interchange, the traffic flow of the main line of the interchange will affect the mental load of the driver. When the driver enters the main line of the interchange, the cubic model has the best fitting effect on the traffic flow at the entrance section and the principal component F₂, showing a cubic curve with an upward opening. The valley point of the curve appears at the level 2 traffic flow, and the model is significant. When the driver drives out of the main line of the interchange, the linear model has the best fitting effect on the traffic flow at the exit section and the principal component F₁, and the model is significant.