Poly(3-azidomethyl-3-methyl oxetane) (PAMMO) was prepared by the azidation reaction of poly(3-mesyloxymethyl-3-methyl oxetane) (PMMMO), which was synthesized by cationic ring-opening polymerization of MMMO for the first time. Two azidation reaction methods of PMMMO were considered to obtain PAMMO securely and efficiently. The thermal decomposition performance of PAMMO was studied by TG/FTIR/MS. The result of TG showed that the thermal decomposition of PAMMO involved two steps. Combined with FT-IR and MS of the escaping gases to investigate the decomposition products of PAMMO, it is found that the first step was mainly corresponding to the thermal decomposition of azide group (-N-3), and the second step was mainly corresponding to the thermal decomposition of the polyether backbone.