Though glycidyl azide polymer (GAP) is a well-known and promising energetic polymer, propellants based on it suffer from poor mechanical and low-temperature properties. To overcome these problems, plasticized GAP-based copolymeric binders were prepared and investigated through the incorporation of flexible-structural polyethylene glycol (PEG) and polycaprolactone (PCL) into a binder recipe under a Desmodur N-100 polyisocyanate (N-100)/isophorone diisocyanate (IPDI) (2 : 1, wt. ratio) mixed curative system. The nitrate esters (NEs) or GAP oligomer were used as energetic plasticizers at various ratios to the polymers. The GAP/PCL binders held the plasticizers much more than the GAP/PEG binders did. The glass transition temperatures (Tg) of segmented copolymeric binders were more dependent on the plasticizer level than the PEG or PCL content. The increase in the plasticizer content decreased the mechanical strength and modulus of binders, while the change of strain was modest. Finally, the NE plasticized GAP-based solid propellants showed enhanced mechanical and thermal properties by the incorporation of PEG or PCL. The properties of GAP/PCL propellants were superior to those of GAP/PEG propellants.