Based on the finite volume method, combined with measured saltwater data and climate model data, a hydrodynamic-salinity model for the dry season of tidal reaches was constructed. Taking the tidal reaches from the dam of the Minjiang Shuikou Reservoir to the estuary, which is severely affected by saltwater, as an example, the upstream characteristics and laws of saltwater intrusion under dry season climate conditions were explored. The results show that during the dry season, the average flow rate of the tidal reaches of the Minjiang River is the smallest, and the climate wind field is in the northeast direction. Under the comprehensive effect, it is most prone to the occurrence of saltwater intrusion. During the dry season climatological spring tide, the mixing of estuary water is strong, and the saltwater intrusion of the South Channel is more intense than the North Channel, which has endangered the Chengmen Waterworks when the saltwater intrusion is the most serious. In the dry season climatological neap tide, the degree of saltwater intrusion is stronger than that of the spring tide, and the South Channel intrusion is much farther than the North Channel. The water stratification is obvious in the tidal fall and the water mixing strongly at the rest of the time. When the saltwater intrusion is severe, the Mawei, Yanshan and Chengmen waterworks are affected during the peak period of water intake.