The invention relates to a double-arm star polylactic acid-polydimethylaminoethyl methacrylate amphiphilic block copolymer and a preparation method thereof. The preparation method comprises the following steps of: synthesizing a star amphiphilic polymer of which the surface contains a cationic group and a core has a hydrophobic structure by an arm-first method, and synthesizing polydimethylaminoethyl methacrylate containing a terminal alkynyl functional group through atom transfer radical polymerization by taking an esterification reaction product 2-butynol ester bromoisobutyrate of butynol and 2-bromoisobutyryl bromide as an initiator, and cuprous bromide and 1,4,7,10,10-hexamethyltriethylenetetramine as a catalyst system; synthesizing polylactic acid containing a nitrine functional group by a lactide ring-opening polymerization method by taking pentaerythritol tribromide azide as the initiator and stannous octoate as a catalyst, and performing an esterification reaction of an obtained product and paraphthaloyl chloride to synthesize polylactic acid of which two ends contain nitrine functional groups; and coupling a polydimethylaminoethyl methacrylate arm with the polylactic acidcontaining the nitrine functional groups to obtain the A2 (B3) 2 type star polylactic acid-polydimethylaminoethyl methacrylate amphiphilic block copolymer. The copolymer prepared by the method has high compatibility and stability, is a potential carrier for simultaneously supporting genes and medicines, and has good application prospect in the aspect of antitumor medicine carriers.