The invention discloses a method for preparing light concrete by municipal solid waste incineration fly ash. The light concrete comprises the following compositions according to material ratio: 330 to 370kg/m<3> of the municipal solid waste incineration fly ash, 450 to 475kg/m<3> of 525# common Portland cement, 325 to 340kg/m<3> of water-granulated slag, 350 to 370kg/m<3> of ordinary clay porcelain granules, 230 to 240kg/m<3> of diabase slag, and 2.7 to 2.9kg/m<3> of admixture. The method uses the municipal solid waste incineration fly ash to prepare the light concrete, can solidify heavy metal in fly ash, thereby achieving the aim of innocent treatment of the municipal solid waste incineration fly ash. The method has a simple process, and the obtained finished product has strength grade of CL40 and apparent density of 1800, and has good macroscopic performance.