With reducing power as the index of investigation, the preparation technology of gallic acid-modified antioxidant peptide from Agaricus Bisporus（GA—ABAP）was optimized by of orthogonal test and in vitro antioxidant activities of GA—ABAP. The results showed that the optimum preparation conditions of GA—ABAP were as followed: volume proportion of GA to ABAP of 5∶1, pH value of 11.0, heating time of microwave of 8 min. The comparison showed that the in vitro antioxidant activities of GA—ABAP prepared in this condition were remarkably improved, the reducing power and DPPH radical scavenging activity of GA—ABAP were 11.16 and 9.83 times higher than those of ABAP, respectively; the metal chelating ability and superoxide anion radical scavenging ability of GA—ABAP were increased by 78.00% and 109.80%, respectively, and suggesting that GA modification may be effective in improving antioxidant activity of antioxidant peptides.