Two-stage reverse osmosis water purifier

The two-stage reverse osmosis water purifier is composed of a first-stage reverse osmosis unit, a water storage tank with an air bag, a second-stage reverse osmosis unit, a pipeline and a control element. The primary reverse osmosis unit is a reverse osmosis water purifier which is publically sold, primary reverse osmosis water prepared from tap water is injected into a water storage tank with an air bag through a water outlet pipeline, the pipeline is provided with a branch, and the primary reverse osmosis water is led out through a valve. And another reverse osmosis water purifier which is publicly sold is used as a secondary reverse osmosis unit. A water outlet pipeline of the primary reverse osmosis unit is connected with a water inlet pipeline of the secondary reverse osmosis unit through a one-way valve. And a water outlet pipeline of the secondary reverse osmosis unit is provided with a valve. A concentrated water outlet pipeline of the second-stage reverse osmosis unit comprises two branches, and one