A hydrodynamic dispersion experiment was done on unsaturated silt loam. Depending on the mass conservation law, formulas to calculate the coefficient of hydrodynamic dispersion were developed. Based on the data of soil water and salt regime measured from the upright soil column, the hydrodynamic dispersion coefficient was calculated, and the relation ship between the hydrodynamic dispersion coefficient and the average pore water velocity was built up. This approach has a clear concept of physics, and the formulas are simple and practical. It can be done with common ap paratus.