Abstract: To solve the safety problem caused by highway drainage, based on the theories of traffic engineering and hydraulics, by analyzing the running characteristics of large scale vehicles on freeway, a simulation model of road surface drainage was constructed by using GAMBIT. The "Spalart-Allmaras" turbulence model in FLUENT software was used to simulate and analyze the safety problem. The variation law of the pressure at the outlet of the single and high density vehicle and the pressure cloud chart were obtained. Results show that the critical headway spacing is 20-100 m and the critical speed of expressway safe operation is from 86 km/h to 29 km/h without continuous moving dam under rainfall intensity of 0.05-0.50 mm/min.