The utility model discloses a chemical desalting system for a phase modifier. The chemical desalting system comprises a raw water tank, a pretreatment system, a reverse osmosis system, a continuous electric desalting EDI system and a plurality of pipelines, wherein the pretreatment system comprises a raw water pump, a bag filter, a micro-oxidation-reduction reactor, an ultrafiltration assembly anda pretreatment water tank which are connected in sequence; wherein the raw water pump is used for pumping raw water in the raw water tank into the bag type filter, and the raw water is sequentially filtered by the bag type filter, the micro-oxidation-reduction reactor and the ultrafiltration assembly and then stored in the pretreatment water tank; a reverse osmosis system; and the continuous electric desalting EDI system is used for deeply desalting the water. The utility model discloses a pretreatment system. A bag filter is adopted for mechanical filtration, granular impurities in water areremoved, a micro-oxidation reduction reactor is additionally arranged, 98% or above of residual chlorine, 99.9% of heavy metal ions, microorganisms, germs and the like in the water body are removed,an ultrafiltration assembly is matched, and it is guaranteed that the pretreated water meets the water using requirement of a reverse osmosis system and runs normally.