The invention provides a method for synchronously detecting L-ascorbic acid, D-ascorbic acid and dehydroascorbic acid in milk powder. According to the method, deoxidized sodium thiosulfate and metaphosphoric acid mixed solution is used as an extracting liquid, active ascorbic acid can be effectively protected, the stability of the ascorbic acid can be improved, light-resistant operation is not required, experimental operation is simple and convenient, and the extract has stable property in 24 hours at low temperature. The chromatographic condition has small damage to a chromatographic column,and the service life of the chromatographic column can be prolonged; a mobile phase is formed by monopotassium phosphate and decyl amine, the resolution can be improved by the concentration of the mobile phase, two geometric isomers can still be effectively separated when the ascorbic acid has relatively high concentration, mutual interference can be avoided, and the content of active ascorbic acid can be accurately detected. The method has the advantages of simple operation, small chemical reagent dosage and good extract stability, has a relative standard deviation of 7.82 percent, can be used for multi-union detection, and has an important meaning for accurately detecting the true content of active ascorbic acid in milk powder.