Abstract: Transportation and diffusion caused by coastal waves hadthe different characteristics due to the flow conditions. Through solvingthe vertical diffusion equations by analytic methods, this paper inferred atheoretical formula for the dispersion coefficient under the combined actionof the wave and the tidal current. It divided the general dispersioncoefficient into six sections, which included coefficients caused by tidalcurrent, Stokes drift, wave oscillation and interaction among them.Meanwhile, it drew a conclusion that the contribution of dispersion effectcaused by coastal waves was mainly produced by Stokes drift, while thecontributions on time-averaged dispersion coefficient by the motion of waterpoints and the wave-current interaction were much smaller. Furthermore, theresults without the tidal current were in agreement with those of numericalsimulation and experiments, which proved the correctness of theoreticalderivation. This paper introduced the variation characteristics that thetime-averaged dispersion coefficient and the amplitude of dispersioncoefficient changed with the variation of wave period and wave height.Meanwhile, we applied the results to vertical circulation and obtained thecharacteristics of dispersion coefficient compared to stokes drift.