RO (Reverse osmosis) for desalination is based on the principle of osmotic pressure. When fresh water and sea water (salt water) are separated by an osmotic membrane, the water molecules in the fresh water will penetrate to the side of the sea water under the action of osmotic pressure. However, when a pressure greater than the osmotic pressure is applied to one side of the seawater, the water molecules in the seawater can penetrate through the osmotic membrane to the fresh water side, and the ions generated by the salt in the seawater cannot pass through the osmotic membrane due to the large volume, so that the water molecules and salt molecules are separated and fresh water is obtained.