Abstract: The safety of mountain highways is not considered enough by current design methods in China, especially for the design of combination of flat and vertical curve. This paper establishes a safety model based on the analysis of vehicle driving safety on the assemble section of flat and vertical curve, taking the critical state of accident as the limit. The influences of various factors on the design indexes are discussed based on the safety model. The Carsim simulation software is used to verify the change trend and the effectiveness of the security model. The safety model calculations and simulations show that the minimum radius is proportional to the driving speed and the highway gradient, and is inversely proportional to the super-elevation and the lateral adhesion coefficient. The maximum slope length is proportional to the pavement rolling resistance coefficient and the slip ratio, inversely proportional to driving speed, highway gradient, and vehicle size, but not affected by the super-elevation. Furthermore, the thresholds of the design indexes of combination of flat and vertical curve considering the operating speed are calculated. Lower highway gradient and larger minimum radius compared to the norm value are recommended.