The invention discloses a method for preparing a phosphate ester polymer by utilizing a rare-earth catalyst, comprising the following steps of: in argon atmosphere in the presence of the rare-earth catalyst, carrying out ring-opening homopolymerization for a five-membered ring phosphate ester monomer or carrying out ring-opening copolymerization for the five-membered ring phosphate ester monomer and a cyclic ester monomer or a cyclic carbonic ester monomer to prepare the phosphate ester polymer. The rare earth catalyst is a halide LnX3, an alkoxy compound Ln(OR4)3 or an aryloxy compound Ln(OAr)3 of rare-earth metal, wherein the rare-earth metal Ln is selected from lanthanide metal, scandium or yttrium; X is a halogen atom; R4 is selected from alkyl groups of C3 to C8; and Ar is selected from phenyl groups or alkyl groups of C1 to C4 in stead of the phenyl groups. The method has the advantages of moderate reaction condition, high efficiency and easy removal of the residual catalyst; and the obtained polymerization product has the advantages of high molecular weight, excellent biocompatibility and degradability, favorable physical and mechanical properties and extensive biological and medical prospect.