Four protocatechuic alkyl esters were synthesized by direct esterification reaction with protocatechuic acid and a series of alkyl alcohols as raw materials, and their structures were identified by infrared and 1HNMR spectrometers. The DPPH assay and ABTS assay were used to determine their free radical scavenging capacity. The Schaal oven test was used to determine their antioxidant efficiency in oil-in-water emulsion. Results showed that the free radical scavenging capacity of protocatechuic acid was enhanced by esterification, and it increased with the increase of alkyl chain length. Esterification of protocatechuic acid enhanced its antioxidant activity in the oil-in-water emulsion. As the alkyl chain length increased, the antioxidant efficiency increased first and then decreased, it reached the maximum at PA-C8. All protocatechuic alkyl esters have a maximum of antioxidant efficiency in rapeseed oil emulsions with the volume fraction of emulsifier 4% and the oil to water ratio 2:8. Compared with emulsions with the addition of protocatechuic acid, the time to reach a conjugated diene value of 1 and the p-anisidine value of 6 was significantly increased.