Salt-tide invasion in the dry season seriously threatens regional water supply in a tidal river area. In this paper, a new model, the withdrawal avoiding saltwater model (WASM), which combines salinity prediction, joint water supply of rivers and reservoirs, and security of water supply analysis, was developed. The model provides an overall idea and decision-making method for security analysis of regional water supply, depending on the withdrawal from tidal river. The water supply for Zhuhai and Macao of the Guangdong-Hong Kong-Macao Greater Bay Area from the Modaomen waterway in the Pearl River Delta was considered as an example. Data of tide, streamflow, and wind factors as well as those of salinity were used. The BP-ANN model for salinity prediction and the function between daily average chlorine and the time exceeding its standard value were well fitted. Upstream streamflow and the time of excessive salinity in the waterway were connected, and the time of withdrawal avoiding saltwater for the streamflow was obtained. The joint water supply of rivers and reservoirs associated with salinity prediction, regional water supply, and demand facing any upstream streamflow during the dry season was analyzed. If only the capacity of withdrawal from the river was considered, the critical demand for the resource supply accounted for 322 million m³ in the dry season. When the regulation of reservoirs was also taken into consideration, the critical demand for the engineering supply accounted for 375 million m³ in the dry season. Then, water supply security was further analyzed, and critical upstream streamflow for various scenarios of water demand in the dry season was identified for given thresholds of risk and critical. The critical streamflow in the dry season accounted for 3372 m³/s when the demand increased to 423 million m³ of local planning water demand. Overall, the larger the demand was, the greater the expected critical streamflow in upstream was. However, the security of water supply is associated with the distribution of upstream streamflow in the dry season.