Water purifier and method for decreasing TDS value of pure water

The invention discloses a water purifier and method for decreasing the TDS value of pure water. The water purifier comprises a raw water inlet, a first reverse osmosis (RO) filter element and a composite filter element; a front filter element is arranged outside the composite filter element, and a second RO filter element is arranged inside the composite filter element; the raw water inlet is connected with a water inlet of a pressurizing pump, and the pressurizing pump is connected with the water inlet end of the first RO filter element through a first water inlet solenoid valve, and meanwhile connected with the water inlet end of the second RO filter element through a second water inlet solenoid valve; and the pure water end of the second RO filter element is connected to the water inletend of the first RO filter element through a second check valve, and a concentrated water outlet of the second RO filter element communicates with an outlet of the front filter element. The compositefilter element is constituted by the front filter element and the second RO filter element, the problems of an excessively large purifier body, cost suddenly-increasing and the like caused by additionally arranging the RO filter elements are avoided, the pure water produced by the second RO filter element is utilized to replace concentrated water in the first RO filter element, the problem of iondiffusion is solved, and thus the TDS of the pure water in the pure water end is not increased.