Abstract: Aimed at the condition that its boiler water salinity is too high in a marine steam power plant, the major factors are analyzed in this paper, and the new design method, adding a demineralizing equipment behind the condenser or the water purification unit, is proposed. According to the analysis, the method of adding a demineralizing equipment behind the water purification unit is feasible, and a new demineralizing equipment is designed. Designing and calculation results show that the water quality in daily water tank improves about 50 times and the water salinity behind the condenser reduces 22 percent. The problem that the boiler water salinity is too high is＇resolved.