3,3'-Bisazidomethyl oxetane-3-azidomethyl-3'-methyl oxetane (BAMO-AMMO) tri-block copolymer was successfully synthesized by azidation of a polymeric substrate containing bromo leaving groups, and an alternative block energetic thermoplastic elastomer (ETPE) was prepared by chain extension reaction. The tri-block copolymer was characterized by Fourier transform infrared (FTIR), 1H NMR, and 13C NMR spectroscopy, X-ray diffraction (XRD), and thermogravimetric analysis (TGA). It was found that the composition of the copolymer is nearly 1?:?1; crystallinity of the copolymer (71.81?%) is less than that of PBAMO (78.30?%). This is due to a partly mixture between soft and hard segments. Kinetic result shows that a crosslinking network is formed after the decomposition of azide group. Tensile strength of alternative block ETPE is 150?% of traditionally synthesized BAMO-AMMO ETPE.