A method for the simultaneous determination of L-ascorbic acid, D-ascorbic acid and dehydroascorbic acid in food was established in this paper by optimizing and improving the sample extraction, extraction agent and mobile phase. Compared with mechanical homogenate, the liquid nitrogen freeze-grinding pretreatment could significantly improve the extraction rate, reduce the dosage of chemical reagents and simplify the pretreatment process for fruit and vegetable samples. 0.019 mol/L sodium thiosulfate was added as a protective agent in the metaphosphoric acid to protect L-ascorbic acid from being oxidized. The results showed that the experiment did not be protected from light, and the extract remained stable within 24 h at 4 oC. The baseline separation for different configurations of ascorbic acid in high concentration could be achieved by using potassium dihydrogen potassium–decylamine as the buffer solution. Ascorbic acid showed a good linear relationship in the range of 0.5 mg/L to 200 mg/L and the detection limit was 0.04 mg/100g. The average recoveries were 99.7%~107.3% and the relative standard deviation was 3.80% (n=5). Consequently, it was of great significance to establish a standardized operating procedure for accurately determining the content of active ascorbic acid in food.