Abstract: Macao and Zhuhai cities are located at Modaomen gate in the Pearl River estuary, where local water resources are scarce and heavily dependent on passing-by water resources. Salt tide is the primary risk factor threatening regional water supply, and joint regulation of basin backbone reservoirs during the dry season is an effective means to ensure water supply security for both regions. By establishing a joint regulation model for the upstream backbone reservoirs of the Xijiang river, a joint regulation model for Macao and Zhuhai pump stations and reservoir groups under the influence of salt tide, this paper calculates and analyzes the water supply gap of Macao and Zhuhai under the influence of upstream water and salt tide, scientifically and rationally optimize the water supply systems of Macao and Zhuhai and propose a water security plan. The research findings indicate that under 97% inflow conditions during the dry season in 2035, the average likelihood of obtaining fresh water at Zhuzhoutou, Pinggang, Guangchang, and Huangyang pump stations will be 32.7%, 13.8%, 0.0%, and 50.0% respectively. To meet the water demand of Macao and Zhuhai and to construct a high-quality, regular and reserve water supply security system for these cities, it is recommended to expand the facilities of Zhuzhoutou, Pinggang, and Huangyang pump stations, and to add approximately 100 million m³ of storage capacity. This study holds significant practical importance for ensuring the long-term water supply of Macau and Zhuhai.