Saltwater intrusion is a serious environmental problem in the Zhujiang River Estuary(ZRE),which threatens the water supply of fifteen million people.The hydrological observations as well as meteorological and tidal forcing in the winter of 2007/2008 were analyzed to examine the saltwater intrusion in the ZRE.The observational results suggest that the maximum vertical difference of salinity can reach 10 in the Humen Channel during neap tide,but is very small in the Hengmen Channel.The vertically averaged salinity from time series stations during spring tide is higher than that during neap tide.A three-dimensional finite difference model was developed based on the environmental fluid dynamic code(EFDC) to study the mechanism of saltwater intrusion and salinity stratification in the ZRE.By analyzing the salt transport and the temporal variation of saltwater intrusion,the authors found that the net salt transport due to the estuarine circulation during neap tide was more than that during spring tide.This caused salt to advance more into the estuary during neap tide.However,saltwater intrusion was stronger during spring tide than that during neap tide because the spring-neap variation in salt transport was small relative to the total length of the saltwater intrusion.The physical mechanism causing this saltwater intrusion was investigated by a series of sensitivity experiments,in order to examine saltwater intrusion in response to river discharge and winds.The freshwater source was a dominant influencing factor to the saltwater intrusion and controlled salinity structure,vertical stratification and length of the saltwater intrusion.The prevailing northeast monsoon during winter could increase the saltwater intrusion in the ZRE.Though the southwest wind was unfavorable to saltwater intrusion during spring tide,it could increase stratification and saltwater intrusion during neap tide. Saltwater intrusion is a serious environmental problem in the Zhujiang River Estuary (ZRE), which threatens the water supply of fifteen million people. The hydrological observations as well as meteorological and tidal forcing in the winter of 2007/2008 were analyzed to examine the saltwater intrusion. in the ZRE. The observational results suggest that the maximum vertical difference of salinity can reach 10 in the Humen Channel during neap tide, but is very small in the Hengmen Channel.The vertically averaged salinity from time series stations during spring tide is higher than that during neap tide. A three-dimensional finite difference model was developed based on the environmental fluid dynamic code (EFDC) to study the mechanism of saltwater intrusion and salinity stratification in the ZRE.By analyzing the salt transport and the temporal variation of saltwater intrusion, the authors found that the net salt transport due to the estuarine circulation during neap tide was more than that during s pring tide.This caused salt to advance more into the estuary during neap tide.However, saltwater intrusion was neap tide because the spring-neap variation in salt transport was small relative to the total length of the saltwater intrusion The physical mechanism causing this saltwater intrusion was investigated by a series of sensitivity experiments, in order to examine saltwater intrusion in response to river discharge and winds.The freshwater source was a dominant influencing factor to the saltwater intrusion and controlled salinity structure, vertical stratification and length of the saltwater intrusion.The prevailing northeast monsoon during winter could increase the saltwater intrusion in the ZRE.Though the southwest wind was unfavorable to saltwater intrusion during spring tide, it could increase stratification and saltwater intrusion during neap tide.