The invention belongs to the technical field of water treatment, relates to a water flow buffer and pure water back-wash technology, and particularly relates to an air bag used for water treatment and a water purification device with the air bag. According to the air bag used for water treatment, and the water purification device with the air bag, the air bag is formed by a high-resistance plastic film and is used for caching water flow and buffering pressure in a water treatment device. With the adoption of the water purification device with the air bag, the air bag is arranged in a filter shell space which is communicated with a filter core pure water port; pure water is cached by the compression of the air bag and discharged by expansion of the air bag, so that the purpose of the invention of back-flowing pure water and back-washing the filter core is achieved. With the adoption of the water purification device with the air bag, the air bag is arranged in the filter shell space which is communicated with a pressure switch in a reverse osmosis water purification device, the problems of frequency opening of the water inlet valve and the frequency starting of a booster pump are solved by caching the pure water and the pressure by the air bag. According to the water purification device with the air bag, the air bag is arranged in a water inlet pipeline of the filter core, so that the purpose of the invention of clearing up water hammer and buffering water inlet pressure is achieved.