Abstract: Chlorosity, water level and river runoff from each saltwater intrusion observation station are collected in recent years (2004-2006) in Modaomen watercourse of Pear River Estary, south China, when the saltwater intrusion occurs.The daily and monthly change of chlorosity and water level at the observation stations are analyzed The correlations among salinity, runoff, estuarine chlorosity and the geometry of estuary are analyzed synthetically, and the model formula between salinity and other hydrological factors (runoff, geometry of estuary and K_x) are deduced.Then, the simulation model for saltwater intrusion length from river mouth in Modaomen watercourse is established Finally, the simulation values of the saltwater intrusion of Modaomen watercourse on January 12, 2006 are calculated based on the model and are very close to the observed values of field.Thus, the maximum length (chlorine: 250 mg/L, upper limit of drinking water) of the saltwater intrusion is calculated based on the modified model parameters of salinity So, in the dry season (usually last December to next March), the model can provide decision making for quick reaction to saltwater intrusion event, and the corresponding measures can be early adopted and the disaster of saltwater intrusion is able to be expected and reduced to the minimization.