Abstract: There is a hidden danger with the shortage of acceleration lane length at freeway interchange.In order to drive sately,the paper consults the traditional method of calculating the length of acceleration lane, and limits the scope of each calculation parameter.Base on above calcolations.An analysis of the effect of road and traffic conditions to acceleration lane length is made the paper presents a new formula in order to adjust the length in theory to meet the demand in practice through the amended vertical slop and traffic volume of main lanes.