The role of dispersion is mainly affected by soil pore structure. Based on the problem, the paper determined the pore size distribution of sandy soil samples by mercury intrusion porosimetry (MIP).Based on the hydrodynamic dispersion theory, one dimensional sand column diffusion experiments were carried out in laboratory. Three sandy soils with different particle size distribution were used as the samples and NaCl solution was used as tracer material in the experiments. The mechanical dispersion coefficient and diffusion coefficient of each soil sample were calculated according to the experiment. The results show that with the increase of d60 or Pu coefficient, the coefficient of molecule diffusion increases little, but the coefficient of machinery dispersion has obvious increase, and the proportion of diffusion role in the whole role of hydrodynamic dispersion become smaller and smaller.The influence of pore structure characteristics on mechanical dispersion is greater than on molecule diffusion.