Seepage velocity of groundwater is a very important hydrogeological parameter of aquifer which can be estimated by tracer technique. There exist shortcomings or restricted conditions in preceding formulae of obtaining the seepage velocity. A tracer dilution physical model of measuring seepage velocity was reestablished according to tracer mass conservation, and a new formula of calculating the horizontal seepage velocity was given by infinitesimal method which modified the generalized dilution formula. Then, the internal relationship between the traditional formula and the new one was discussed. A method to deal with the practical data was given combining with an engineering case. The result indicated that the new formula could get more abundant information of the horizontal seepage velocity in the aquifer, which was much close to the local geological condition and more reliable.
