The invention relates to the field of water treatment and discloses a treatment method and a treatment system for coal water slurry gasified black water. The method comprises the following steps: (1) performing contact of the coal water slurry gasified black water with a Ca(OH)2 solution; (2) performing flocculent precipitation on the product obtained in the step (1) and a flocculant to obtain precipitate and a supernate; (3) performing contact on the supernate obtained in the step (2) with CO2; (4) removing suspended solids and/or colloids in the produced obtained in the step (3); and (5) removing soluble mineral ions in the product obtained in the step (4). By adopting the method provided by the invention, harmful ions such as Cl<->, Ca<2+> and Mg<2+> in lye can be removed effectively, the reuse rate of the lye is increased, discharge of the lye is reduced, and the operating cost and investment of a sewage disposal station are lowered.