With laboratory testing rig and testing program designed, the fine sand, medium sand and coarse sand are chosen as test material respectively, and the salt solution is chosen as tracer agent, then the seepage velocity is calculated through the relationship between the conductivity of tracer agent in different test material and time. The value of seepage velocity calculated through traditional dilution formula in all sand layers is a little larger than that by testing discharge method. The specific value of them is constant in the same hydraulic gradient, while it is imparity in different sand layers. The dilute tracer correction coefficient is proposed as well as the modified dilution formula, meanwhile the experimental value of dilute tracer correction coefficient is given.