The invention belongs to the technical field of metal smelting, and particularly relates to a harmless treatment technology for cadmium alkali residues. A method comprises the steps of (1) putting the cadmium alkali residues into a reactor, adding deionized water, stirring the cadmium alkali residues with deionized water to be dissolved, and recycling insoluble cadmium, cadmium oxide and a part of insoluble impurities; (2) adding Na2S into filtrate for reaction, and recycling zinc-containing components; and (3) finally, adding a decolorising agent, carrying out decoloration till the filtrate is colorless, and carrying out concentration treatment on the filtrate, so as to obtain an NaOH concentrated solution. According to the method, the technology is simple during the entire process, investment is low, all the resources are recycled, secondary pollution is avoided, resource waste is reduced, the environment is protected, and economic benefits of enterprises are increased.