Tri-functional random copolyether of NIMMO and THF(tri-functionality NIMMO-THF copolyether) energetic binder was synthesized by the ring opening polymerization of NIMMO(3-nitratomethyl-3-methyloxetane) and THF(tetrahydrofuran) using trimethylolpropane(TMP) as initiator,boron trifluoride ether complex BF₃·OEt₂ ascatalyst. The products were characterized by infrared spectroscopy, nuclear magnetic resonance and differential scanning calorimetry(DSC). Taking tri-functionality NIMMO-THF copolyether energetic binder curried and synthesized by hexamethylenediisocynate (HDI) as curing agent, a series of elastomers were prepared according to the molar ratio of ─NCO of hexamethylenediisocynate (HDI) to ─OH of tri-functionality NIMMO-THF (R value) as R=0.8, 0.9, 1.0, 1.1, 1.2 respectively. The mechanical properties and thermal properties of elastomers were investigated. The results show that the temperature of exothermic peak of the tri-functionality NIMMO-THF copolyether energetic binder obtained by DSC at 10 ℃·min^-1 is 216 ℃. When R value is 1.1, the tensile strength of the elastomer is the highest, which is 1.32 MPa and the temperature of exothermic peak of the elastomer obtained by DSC at 10 ℃·min^-1 is 203 ℃.