A method for preparing terpolymer of poly (biphenyl sulfone ether) and poly (ether sulfone) comprises: adding high-temperature organic solvent into a flask, stirring and heating to 80° C., adding 4,4′-dichlorodiphenylsulfone,4,4′-Bis(4-chlorophenyl)sulfonyl-1,1′-biphenyl and 4,4′-dihydroxydiphenylsulfone with solid content of 20-35%; stirring until monomers are completely dissolved, heating to 100° C., adding salt-forming agent and xylene; stirring while heating so that salt-forming reaction begins in the system, wherein the system temperature is controlled between 190° C. and 210° C.; when the amount of water discharged reaches the theoretical value, the first stage of salt-forming reaction is finished; heating the system to 230-236° C., and maintaining for 3-4 hours to obtain polymer viscous liquid; and refining the polymer viscous liquid to obtain a terpolymer containing different structural units in the molecular chain, wherein the Tg of the terpolymer can be regulated by changing the ratio of the two dichloro-containing monomers.