Abstract: Based on the well calibrated 1D and 2D coupled numerical model for rivers and coastal waters of Qinhuangdao, the influences of the Yanghe reservoir flood discharges on water quality in coastal waters were discussed. Due to COD concentration in the reservoir is slightly higher than COD standard concentration of coastal water and considerably lower than COD concentration of the river, which leads to the daily average concentration of COD and over-standard area of COD concentration will increase in the estuaries at the beginning of reservoir water release. Then with the continuous release of reservoir water, the daily average concentration of COD and over-standard area will gradually decrease, and finally tend to be stable. Hence, the water quality of coastal water will be significantly improved. Under the same COD concentration in reservoir, the more discharge of reservoir water will carry more downstream pollution to the coast, the larger of over-standard area in coastal water, and the relationship of reservoir flood discharge and over-standard area can be well fitted by a linear function. In the case of the same pollutant flux into the coastal water, reducing pollutant concentration will improve water quality of coastal water.