To improve the security of long and steep downgrade in mountain highway,we establish a temperature model of truck brake speed on long and steep downgrade with the theory of energy conservation.Some parameters of the model have been modified through field tests and mathematical statistics.The critical gradient Ⅰ and Ⅱ concept of long and steep downgrade is proposed,and the security risk rating index of long and steep downgrade is proposed.The results show that the critical gradient based on the brake temperature is the control gradient of the smooth transition section design in the long and steep downgrade section of freeway in mountain areas.Brake temperature model for truck owners is as an effective means of rapid assessment of road safety.It can quantify the security risk rating of long and steep downgrade,which is helpful for p the selection of hedge lane location and the implementation of other security support measures.