The invention relates to the field of utilization of fly ash, particularly to an Al-SBA-15 mesoporous molecular sieve, a preparation method and applications thereof, and a denitration catalyst, a preparation method and applications thereof. The preparation method comprises: 1) mixing fly ash and an acid solution, carrying out an acidolysis reaction, and filtering to obtain an aluminum salt solution and silicon-rich filter residue; 2) mixing the silicon-rich filter residue, an alkali and water, carrying out an alkali dissolution reaction, and filtering to obtain a silicate solution; 3) adjusting the pH value of the silicate solution to achieve an acidic pH value, and adding a polyethylene oxide-polypropylene oxide-polyethylene oxide triblock copolymer into the solution to prepare a synthetic mother liquor; 4) performing a hydrothermal crystallization reaction on the synthetic mother liquor to obtain an SBA-15 molecular sieve; and 5) mixing the SBA-15 molecular sieve and an aluminum saltsolution to obtain the Al-SBA-15 mesoporous molecular sieve. The Al-SBA-15 mesoporous molecular sieve provided by the invention has a microporous and mesoporous double-pore structure and has good application prospect.