Through the analysis and estimation of water demand and supply capacity in Tianjin, predicted by 2020, the gap between the supply and the demand of water resources is about 300 million m³, which needs the seawater desalination to fill. The scale of desalination project is expected to reach 800 000 tons/day. According to the purposes of seawater desalination, using the proportion assigned method and the Cobb-Douglas production function, the calculated economic value of seawater desalination for domestic water, industry production and comprehensive utilization of concentrated seawater are 39. 75 yuan/m³, 96. 8 yuan/m³ and 270 million yuan/year, respectively. The research results aims to fully understand the value of seawater desalination, and provides references for the scientific decisions of its development.