The title compound, 5-azido-1, 2, 4-triazolyl-5-acetic acid (ATAA), was synthesized for the first time by Sandmeyer-reaction in a mixed system (including sodium nitrite, sulfuric acid and sodium azide) from 5-amino-1, 2, 4-triazolyl-5-acetic acid. Its chemical structure was fully characterized via NMR, IR, MS and elemental analyses. Typical TG and DSC curves indicate the thermal behavior of ATAA includes one dehydrating crystal water stage, one melting stage and one obvious melting decomposition stage. The peak temperatures at each stage are 85.6 ℃, 168.0 ℃ and 177.9 ℃, respectively. In addition, a novel polynitro-azole energetic compound (5-azido-3-trinitromethyl-1H-1, 2, 4-triazole) was designed and calculated. The standard enthalpy of formation, the calculated density as well as the detonation velocity are 449.62 kJ·mol^-1, 1.91 g·cm^-3 and 9096 m·s^-1, respectively.