Disclosed is a seawater desalination method. The method includes reading parameters representing properties of one or more membranes constituting a membrane combination to be applied to each membrane module; calculating water flux and salt flux of the membrane modules based on the read parameters; calculating a volumetric flow rate imbalance and a total permeate flow rate of the membrane modules based on the calculated water flux and salt flux; and determining a membrane combination having a minimum volumetric flow rate imbalance and a maximum total permeate flow rate based on the calculated volumetric flow rate imbalance and total permeate flow rate. Thus, it is possible to achieve arrangement optimization of reverse osmosis membrane modules.