Abstract: Based on the theoretical analyse and model test, the mechanism of siltation in thedredged harbour basins on tidal estuaries is explicated and a method to calculatethe siltation is developed. The effect of mouth direction of dredged harbour basins on thesiltation is also discussed. By using the method given in the paper, the siltation of Baoshansteel factory harbour basin is calculated and a good agreement with the field measurementis obtained.