Abstract: It is often meet in the process of designing the expressway in the mountainous area to design long steep downgrade section. But the current route design specification doesn't have the definition criterion of long steep downgrade sections. Starting from the driver's driving behavior and driving needs, the running speed was taken as the macro performance of driving behavior to analyze the performance of driving behavior in this article. Some experimental data were collected from the real vehicle test on 20 truck drivers in the mountainous expressway long steep downgrade section. By analyzing the data, the correlation between the speed with the average longitudinal slope and the longitudinal slope length was found, and a prediction model was established for the truck operating speed. And then the maximum longitudinal slope length under the condition of different longitudinal slope was got at 60 km/h design speed. The conclusion can provide a reference for the designing and operation management of expressway long steep downgrade in mountainous area.