The oxidized starch with high carbonyl content disclosed by the invention has a carbonyl content of 15.5-55.4% and a molecular weight greater than 25,000. The invention also discloses a preparation method of oxidized starch with high carbonyl content. The method is to prepare starch emulsion with water concentration of 10-30%, then heat up to 60-90°C, stir and gelatinize for 0.5-2 hours, and then cool to At room temperature, add 50-150 parts of water and stir evenly; slowly drop in hydrogen peroxide with a molar ratio of 0.2-4 to starch, stir and react at a temperature of 5-50°C for 0.5-72 hours; Add 200 parts of water to the mixture and stir evenly, pour off the supernatant after standing for stratification, centrifuge the emulsion, and dry the obtained solid. The oxidized starch prepared by the method has high carbonyl content, good thermal stability, low preparation cost, high yield and simple process, and can avoid the problem of serious molecular weight drop caused by preparing oxidized starch in an acidic or alkaline medium.