The invention relates to a preparing method of a high polymer containing an oxazolidinone ring. The preparing method includes the steps that polyhydric alcohol and diisocyanate react in the presence of organic solvent to prepare an isocyanate-terminated prepolymer; in the presence of organic solvent remaining isocyanate groups in the isocyanate-terminated prepolymer are completed blocked to obtain an isocyanate blocked substance; the isocyanate blocked substance and aliphatic epoxy resin react in the presence of a catalyst, the high polymer containing the oxazolidinone ring is obtained through reduced pressure distillation after reaction is completed, wherein the molar ratio of additive amount of aliphatic epoxy resin to remaining isocyanate groups in the first step is (1.5-2.5):1. The oxazolidinone polymer is obtained through a three-step method, in the method, the reaction rate can be effectively controlled, and it is avoided that gel is generated in the product due to excessively intense reaction. The synthesized oxazolidinone polymer is good in performance, mild in viscosity and high in temperature resisting index and has important significance in researching H-level oxazolidinone solvent-free insulating paint.