The invention relates to a method for preparing geopolymer by using sludge ash. The preparation method comprises: by taking sludge ash and metakaolin as raw materials, generating a sludge ash-based geopolymer under activation of a NaOH-water glass alkali-activator solution. The preparation method is simple and feasible in process, and not only can save resources, but also can realize resource utilization of sludge, thereby turning waste into wealth and killing two birds with one stone.