The operation of reservoir will change the downstream runoff and affect the salinity of tidal river because of the effect of profiting regulation. Based on combining the geological with hydrological data, the paper established one-dimensional flow-salinity coupling model so as to study the salinity's variation in downstream section after the operation of a large reservoir in Zhejiang Province.The result showed that after the operation of reservoir, the essence of saltwater intrusion will not change utterly.The reservoir regulation could have beneficial effect on the reduction of saltwater intrusion.