In the construction of expressway in mountain area, the contradiction of the maximum longitudinal slope with engineering's volume and cost is very prominent. If the construction standard was executed strictly, the engineering volume of high fill and deep excavation will be very big, many long span bridges and tunnels must be constructed from the point of vehicle dynamics, and the maximum longitudinal slope and its length of mountain expressway under the condition of vehicle's driving down are analyzed. The concepts of ideal maximum longitudinal slope, general maximum longitudinal slope and the limitation maximum longitudinal slope are put forward. And their length is determined to provide a reference for highway designer.