To ensure safety and running smooth of the railway trains, according to the Rail- way Track Design Standards GBJ90-85,the maximum difference in gradients of the adjacent slope-sections used in the vertical-sectional design of the railway tracks had been defined clearly as ： the smaller difference in gradients should be used in the designs for connecting the adjacent slope-sections, and the maximum value should not exceed the limiting slope in the heavyhaul direction. The value of Ai,?* must be used so as to ensure running smooth，un-derail, no uncoupling and no coupler broken. This value has been further researched in this paper.