In order to accurately grasp the relationship between the geometric parameters of different roads and the critical safety driving speeds of a curve， a mathematical model for calculating the critical safety speed of multi dimension road design is established.Through the Trucks im software， a large-scale vehicle was used to establish the simulation test of dif-ferent ramp turns upper elc vation， radius of the curve and safe driving speed， and the simu-lation test scatter was fitted and verified by the least squares method.Then， the experimen-tal data under the influence of interaction are obtained by orthogonal simulation experiment. The binary polynomial regression mathematical model for road geometry is fitted through the curve.The design safety speed of the general value and the limit road conditions in the"Design Specification for Highway Alignment"is linear compared with the calculated safe speed of the model， and the safety speed obtained by the calculation model is represented by other traditional computational model and the Lee model are compared.The results show that the mathematical model is effective and reliable， and the accuracy error is the smallest under a certain condition.With the increase of the ramp turns upper elc vation， radius of the curve， the critical safety speed gradually increases with the design safety speed.