On the basis of review of known works carried out on ocean water (Persian Gulf) it is shown that preliminary nanofiltration is an effective method of preventing sulphate scaling in desalination systems. The process of nanofiltration purification of Caspian Sea water was investigated by means of computational experiments on a computer. High values of selectivity of NF-200-400i membranes for scale-forming components are established. It is shown that preliminary nanofiltration of Caspian water reliably prevents sulphate deposits at subsequent stages of reverse osmosis and thermal desalination. There is a possibility to increase desalinated water yield, to organise distillation process in the area of high temperatures and, as a result, to reduce desalination costs. Technological schemes of desalination and desalination of Caspian Sea water with nanofiltration pre-treatment are proposed.