The invention provides a method and a system for sea water desalination. The method comprises the steps of firstly carrying out forward osmosis treatment on raw material liquid to be desalted, wherein water in the raw material liquid penetrates through a membrane and enters a draw solution at the low osmotic pressure side from the low osmotic pressure side of a forward osmosis membrane; pressurizing the draw solution, then carrying out reverse osmosis treatment so as to obtain desalted fresh water at the low pressure side of a reverse osmosis membrane; and finally carrying out membrane distillation treatment on the desalted fresh water, wherein the water vapor penetrates through the membrane and is condensed so as to obtain the purified fresh water. Correspondingly, the system for sea water desalination comprises a raw material liquid tank, a draw solution tank, a water collecting tank, a forward osmosis membrane component, a reverse osmosis membrane component, a membrane distillation component, a peristaltic pump, a high pressure pump, a heater, and a condenser. The method and the system consume less energy, steps of forward osmosis and membrane distillation do not need impressed pressure, contamination to the membrane is low, and the contaminant is easy to remove.