On the basis of mass and heat energy balance of low temperature multiple-effect distillation （LT-MED） seawater desalination system, the optimization mathematical model of power and water cogeneration system with parallel/cross flow is developed in which condensed water flash distillation and seawater preheating are considered. The minimum cost of unit water is the optimization target. The optimization model of power and water cogeneration system is solved and analyzed by computing program which is carried out with Matlab toolboxes. The influence of number of effect, the amount of heating steam, system scale on the cost of unit water is analyzed. In the meantime, the costs of unit water with power and water cogeneration, equal temperature difference design with power and water cogeneration and equal temperature difference design with no power and water cogeneration are compared. The results indicate that the cost of unit water with power and water cogeneration is 87. 54% lower than that of equal temperature difference design with no power and water cogeneration with the same parameters. Power and water cogeneration is an effective way to reduce the cost of unit water.%在对带有冷凝水闪蒸和海水预热的低温多效蒸发海水淡化系统进行物料和热量衡算的基础上，建立了相应海水淡化系统并叉流流程下水电联产系统优化的数学模型．该模型以单位产量淡水成本最小为优化目标，利用Matlab工具箱编制了求解程序，对水电联产系统数学优化模型进行了求解，分析了效数、加热蒸汽流量和系统规模对单位产量淡水成本的影响，并将水电联产优化设计、水电联产等温差设计和无水电联产等温差设计3种方案下的单位产量淡水成本进行了比较．计算结果表明：参数相同情况下，水电联产优化设计和无水电联产等温差设计相比，最大可节约淡水成本87．54％．水电联产低温多效蒸发是降低海水淡化成本的有效途径．
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