The invention relates to a washing desalination system and method for waste incineration fly ash or kiln dust. The invention aims at providing the washing desalination system and method with multiplegrading manners, good desalination effect and low cost. The washing desalination system comprises a conveying system and multiple stages of elution systems, wherein the elution systems include a first-stage elution system, a second-stage elution system and a multi-stage elution system; the content of chlorine salt in to-be-processed fly ash or kiln dust is more than or equal to 4% and less than orequal to 10%, and the first-stage elution system is adopted when the chlorine content is more than or equal to 2% and less than or equal to 5%; the content of chlorine salt in to-be-processed fly ashor kiln dust is more than or equal to 10% and less than or equal to 20%, and the first-stage elution system and the second-stage elution system are sequentially adopted when the chlorine content is more than or equal to 5% and less than or equal to 10%; and the content of chlorine salt in to-be-processed fly ash or kiln dust is more than or equal to 20%, and the first-stage elution system, the second-stage elution system and the multi-stage elution system are sequentially adopted when the chlorine content is more than or equal to 10%.