A nuclear heating plant is a cogeneration nuclear heating plant, which provides industrial steam for the chemical industrial park to satisfy the steam demand of enterprises. Meanwhile, the remaining capacity can be used for power generation, so as to alleviate the local power shortage. As the condensed water of the externally supplied steam is not recovered, and the freshwater source is insufficient, it is necessary to build a seawater desalination plant and a demineralized water production plant to provide a large amount of demineralized water for the heating system. According to the raw seawater quality, requirements of produced water quality and technical & economic analysis, the main treatment process flow of seawater desalination system is: air flotation tank → horizontal mechanical filter → ultrafiltration membrane → seawater reverse osmosis membrane → freshwater reverse osmosis membrane. The main treatment process flow of demineralized water production system is: cation exchanger → anion exchanger → mixed ion exchanger. Combined with the engineering practice, this paper expounds the process system design of large-scale seawater desalination and demineralized water production from the aspects of seawater desalination process comparison and selection, reverse osmosis membrane seawater desalination process system design and process equipment configuration, etc. The system improves the guarantee rate of desalinated seawater and demineralized water supply, and realizes the emission reduction of industrial wastewater, which can provide reference for the process design of large-scale seawater desalination and demineralized water production in subsequent nuclear heating projects or nuclear power projects.