The invention discloses a method of modifying hollow glass microbeads with an ionic liquid and a flame-retarding thermoplastic polyurethane elastomer with the modified hollow glass microbeads as a flame retardant. The method includes the steps of: 1) treating the hollow glass microbeads with a hydrochloric acid solution; 2) after the acid treatment, repeatedly washing and drying the hollow glass microbeads; 3) activating the hollow glass microbeads, adding a silane coupling agent, and performing mechanical stirring reflux; 4) stopping the reaction and cooling the reaction product to room temperature, and performing suction filtration with acetonitrile, ethanol and deionized water and drying the reaction product; 5) adding the ionic liquid to the silanized hollow glass microbeads and performing mechanical stirring reflux; 6) stopping the reaction and cooling the reaction product to room temperature, performing suction filtration successively with ethanol, a mixed solution of ethanol and deionized water, and ethanol, and drying the reaction product. By means of the hollow glass microbeads modified with the ionic liquid as the flame retardant for preparing flame-retarding thermoplastic polyurethane (TPU) elastomer, not only is oxygen index of the composite material improved, but also the vertical combustion performance, which reflects melt-dripping resistance, is improved, so that heat release rate is reduced and smoke is significantly inhibited.