Disclosed is a preparation method for an antitumor drug AP26113 (I). The method comprises the following preparation steps: cyclizing N-[2-methoxyl-4[4-(dimethyl amino)piperid-1-yl]aniline]guanidine and N,N-dimethylamino acrylate, condensing N-[2-methoxyl-4[4-(dimethyl amino)piperid-1-yl]aniline]guanidine and 4-(dimethyl phosphitylate)aniline, and chlorinating N-[2-methoxyl-4[4-(dimethyl amino)piperid-1-yl]aniline]guanidine and a chlorinating agent, sequentially, so as to prepare AP26113 (I). The preparation method adopts easily-obtained raw materials, causes few side reactions, and is economical, environmentally friendly, and suitable for industrial production.