Mixed bed is an ion exchange device that mixes anion and cation exchange resins in a certain ratio in the same exchange column. The resin layer is evenly mixed, and the Yin and Yang resins are staggered tightly. Each pair of cation resin and anion resin particles is similar to a group of multiple beds, so the mixed bed can be regarded as a series of multiple beds. Since the hydrogen ions and hydroxide ions that enter the water by reverse osmosis after mixed ion exchange can immediately generate water molecules with a lower ionization degree, it is rarely possible to form cations or anion exchange counter ions, so that the exchange reaction is very thorough. Therefore, the water quality of the mixed bed is better than the double bed of yin and yang in series, and the effluent purity is higher