The invention discloses an isoflavone derivative modified by acetylaminoacid benzyl ester of a general formula Ia-q, in which R1, R2 and R3 are all CH2CO-AA-OBzl or both R1 and R2 are all CH2CO-AA-OBzl, while R3 is H, wherein the AA in the CH2CO-AA-OBzl is selected from glycin, lactamine, phenyl alanine, tryptophan, isoleucine, leucine, valine, aspartic acid and glutamic acid. The invention also discloses a preparation method of the isoflavone derivative modified by the acetylaminoacid benzyl ester of the general formula Ia-q. The invention adopts an S180 mouse model to evaluate the antitumor effects of the isoflavone derivative modified by the acetylaminoacid benzyl ester of the general formula Ia-q, thus indicating that the isoflavone derivative has excellent antitumor activity and can be used as an antitumor agent.