To establish the correlation between the microstructure and mechanical properties of high nitrate plasticized polyethylene glycol （PEG） based polyurethane in nitrate ester plasticized polyether （NEPE） propellant， biuret triisocyanate （N-100） was used as a multifunctional curing agent and mixed with nitrate ester plasticized PEG for curing to prepare PEG elastomers with curing parameters ranging from 1.2 to 1.7. The microstructure of PEG elastomer crosslinked network was studied by uniaxial tension， X-ray diffraction， low field nuclear magnetic resonance and equilibrium swelling test methods. Furthermore， the effects of different network chain structures on the mechanical properties of PEG elastomer were analyzed. The results show that the PEG elastomer is amorphous due to its high plasticized properties. Meanwhile， the total ratio of suspended tail chains to free chains is more than 85%， the structural integrity of the crosslinked network is low， and the elastomer exhibits high elongation， low tensile strength and low initial modulus. Secondly， all the tensile strength and initial modulus of elastomer are positively correlated with the crosslinked chain density. The maximum elongation increases first and then decreases with the increase of physical temporary entanglement chain density. CU-5 elastomer with a curing parameter of 1.6 has the most complete cross-linked network. At the same time， the corresponding tensile strength is 0.80 MPa， and the maximum elongation is 1456%， which indicates that the mechanical properties are the best. Finally， the chain density measured by equilibrium swelling method and low field nuclear magnetic resonance satisfy the magnitude relationship of ν_L，A<ν_s<ν_L，A+B.