The present invention provides a noise-reducing aerated concrete building block and a preparation method thereof, wherein the noise-reducing aerated concrete building block is prepared from the following raw materials by mass: 30-45 parts of cement, 120-150 parts of lime, 10-20 parts of gypsum, 45-60 parts of sand, 400-500 parts of fly ash, 0.5-1 part of aluminum powder, 1-2 parts of a foam stabilizer, 1-2 parts of a water-reducing agent, and 18-27 parts of water. The preparation method comprises: 1) carrying out mixing-material preparing; 2) casting the slurry into a mold, and carrying out static stopping; and 3) cutting, carrying out autoclaved curing in an autoclave, discharging out of the autoclave, and demolding. According to the present invention, the raw material ratio and the process parameters such as the raw material casting, the autoclaved curing and the like during the preparation process are regulated to prepare the noise-reducing aerated concrete building block with characteristics of good pores, effective noise absorbing, strength grade of A3.5, dry density grade of B06, good work performance, and good durability.