Effect of electric or magnetic field on desalination efficiency was studied by molecular dynamics simulation. A restricted double-walled carbon nanotube (CNT) was selected, then strong and weak external field regime were applied on simulated system. Results of simulation showed that strong electric field increases water flow rate two times than untreated system. Magnetization of water decreases water flow rate because of the increase in number of hydrogen bond. In addition, it was observed that applying electric field or magnetization of water completely rejects the ions. In fact, electric field moved back Na+ ions because of the force acting on the ion opposite to the direction of reverse osmotic forces. Magnetization of water leads to a more stable hydration shell around the Na+ ion that prevents passing through the carbon nanotube. In comparison with a single-walled CNT, considering a double-walled CNT enhances the experimental agreement of simulated water flow rate.