Abstract: Based on hydrodynamic and pollutant transport model, the transport processes of chemical oxygen demand (COD) in Caofeidian sea area under in flood and dry seasons are simulated.Numerical results show that COD concentration is determined by seasonal discharges and pollutants, while regional tidal current controls its dynamic transport process.In eastern Caofeidian sea area, the area of high concentration of COD exists in Erxiedazhuang estuary in both flood and dry seasons with maximum concentration of 2.2 mg/L, while concentration of COD in Daqinghe estuary and Erluanhe estuary decreases significantly in dry season.Pollutants expand to the sea along SW direction during flood tide and transport to the estuary again during NE ebb tide.In contrast, the seasonal variation of COD concentration distribution in western Caofeidian sea area is not significant, and COD mainly concentrated in Shahe estuary and drain outlet of Sanyou Chemical Industries Company.Effected by NW-SE direction reciprocating tidal current, COD transports toward estuary during flood tide while expand to the sea during ebb tide.