Composite retarded acid

The invention discloses composite retarded acid, and belongs to the technical field of sandstone formation acidizing. The composite retarded acid comprises the following components in percentage by weight: 16-18% of methyl formate, 5-6% of ammonium fluoride, 9-11% of hydrochloric acid, 0.8-1.2% of a corrosion inhibitor, 1.2-1.7% of a ferric ion stabilizer, 1.3-1.6% of a clay stabilizer, 1.2-1.6% of a discharge aiding agent, 0.8-1.2% of an emulsion breaker and the balance of water; methyl formate can be hydrolyzed in acid liquor to generate formic acid, and formic acid can then react with ammonium fluoride to generate needed hydrofluoric acid; when retarded acid enters the stratum, the amount of formic acid generated through hydrolysis of methyl formate is very small, the amount of hydrofluoric acid generated through reaction of formic acid and ammonium fluoride is relatively small, the reaction rate among acidic rock is further retarded; enough time is provided for the retarded acid for entering deep formation, when the retarded acid enters the deep formation, the hydrolysis reaction speed of methyl formate is gradually increased along with gradual consumption, the amount of newly generated formic acid is gradually increased, correspondingly, the reaction rate among acidic rock is increased, and the extraction efficiency of underground reservoirs is further improved.