The invention discloses a deoiling agent for oilfield binary composite displacement output sewage and a preparation method thereof. The deoiling agent comprises acryloyloxy ethyldimethylamine, chloroalkane, acrylamide, N,N'-methylene-bis acrylamide, ammonium persulfate and sodium sulfite. The preparation method comprises the following steps: reacting the acryloyloxy ethyldimethylamine and chloroalkane (CnH2n+1Cln=1-12) to generate a cationic monomer; and mixing the acrylamide and N,N'-methylene-bis acrylamide, adding deionized water by three times while stirring, completely dissolving, adding the cationic monomer, respectively adding ammonium persulfate and a sodium sulfite solution, and stirring at the constant temperature of 45 DEG C to react for 3-5 hours, thereby obtaining the deoiling agent. The deoiling agent is simple to prepare and convenient to use, and has the advantages of low chemical consumption, low cost and high deoiling ratio (higher than 90%); the separated recovered oil does not influence the subsequent crude oil dehydration; and therefore, the deoiling agent can be widely used in the oilfield binary composite displacement output sewage deoiling technique.