When the clear distance between the ramp and the longitudinal slope of the expressway is small, vehicles are easy to be affected by the diverging and merging when going up and down the slope, and the variation of speed is large, which is not conducive to driving safety. Based on the minimum clear distance between the ramp and the longitudinal slope, UC-Win /Road was used to establish the Inlet-uphill and downhill-exit section simulation model, and the experiment was carried out with Forum8 driving simulator and data were collected. The control variable method was used to adjust the characteristic indicators such as the mixing rate of large vehicles and the proportion of diverging and merging to study the real-time variation law of vehicle speed in longitudinal section. A velocity measurement model was built to quantify the relationship between the characteristic indicators and the velocity, and a single factor sensitivity analysis was conducted for the characteristic indicators. The results show that the change of ratio of diverging and merging and the mixing ratio of large vehicles have significant influence on the speed change, and the velocity magnitude and change trend of different vehicle types are quite different. The change of mixing ratio of large vehicles has a great influence on the running state of vehicles in the uphill section, while the change of diverging ratio has a great influence on the running state of vehicles in the downhill section. When the mixing ratio of large vehicles and the ratio of diverging and merging is 30%~40% and 10%~15% respectively, the velocity fluctuation is the largest and the sensitivity is strong. The research results can be used to provide a theoretical basis for vehicle operation management and improvement of safety measures when the clear distance between ramp and the longitudinal slope is small. [ABSTRACT FROM AUTHOR]