The invention provides a method for preparing a 4A type molecular sieve used for a detergent. The method comprises the following steps of: mixing desiliconization liquid serving as a silicon source and sodium aluminate coarse solution serving as an aluminum source in a certain proportion, wherein the desiliconization liquid is generated in a pre-desiliconization process section in a process of preparing aluminum oxide from high-alumina fly ash, and the sodium aluminate coarse solution is obtained in a clinker dissolution process section in the process of preparing the aluminum oxide from the high-alumina fly ash; stirring the mixture at a low temperature; and raising the temperature and crystallizing the mixture to form the 4A type molecular sieve. The 4A type molecular sieve prepared by the method has the advantages of high exchange capacity of calcium, small granularity and high whiteness and is an excellent washing auxiliary detergent. The method has the advantages of simple process, cheap raw material, no waste liquor treatment and capability of greatly reducing the preparation cost of the 4A type molecular sieve.