Abstract: Owing to are some questions about the traditional dilution formula and the generalized dilution formula deduced by predecessors,a new dilution formula was derived to calculate seepage velocity with tracer method,based on the condition that there existed vertical flow in the borehole and a reasonable physical model was adopted,considering the seepage velocity as a function of tracer dilution time.When there was vertical flow in borehole,by the comparison between results of three formulas and the error analysis,it was shown that the greater the relative error of the seepage velocity calculated by the new deduced formula and traditional dilution formula was,the larger the difference of the vertical flow velocity of the borehole′s upper and lower parts was,which was in accordance with the practical situation.So the results calculated by the new formula are more credible.