With the rapid development of coastal cities, the conventional surface water and groundwater can no longer meet the social and economic development. Taking Weihai Power Plant of Huaneng Power International Inc. as an example, based on the full-cost theory, a full-cost analysis model for seawater desalination is established, and the resource cost, project cost and externality cost are calculated. The resource cost is considered from the value of utility and scarcity. The project cost includes the construction cost and initial investment cost. The effect of seawater desalination on the ecology is measured using the pollutant equivalent method. The full cost of desalinated water of seawater in Weihai Power Plant is 7. 166 yuan/m³, of which the resource cost is 0. 123 yuan/m³, the project cost is 6. 597 yuan/m³ and the externality cost is 0. 446 yuan/m³. Compared with the prices of the current tap water and externally supplied water, there is no obvious price advantage for desalinated water. However, due to its stable water volume and safe water quality, it still has certain application potential in the urban water supply system in the future. Finally, in order to improve the competitiveness of desalinated water in the water supply system of coastal water-deficient cities, the relevant policy suggestions are put forward.