Hexanitroazobenzene(HNAB) was prepared via nucleophilic substitution reaction and oxidation reaction using picryl chloride as raw materials. The intermediate and final product were characterized by MS, IR and ¹H NMR. The reaction conditions were optimized. The nucleophilic substitution reaction conditions are determined as: hydrazine sulfate 2.0 g, 2, 4, 6-trinitrochlorobenzene 7.6 g, methyl alcohol 15 mL, triethylamine 6 mL, reaction time 24 h under 45 ℃. The oxidation reaction conditions are determined as: 1, 2-bis(2, 4, 6-trinitrophenyl)hydrazine 2.0 g, iron(Ⅲ) nitrate nonahydrate 7.1 g, cumulative grinding 8 h. The total yield is 68.5%. Thermal behaviors of HNAB were studied by DSC and TG. The thermal decomposition mechanism of HNAB is classified as random nucleation and growth with n=1. The activation energy, pre-exponential factor and mechanism function of the thermal decomposition process of HNAB obtained by the Ozawa′s method are 123.48 kJ·mol^-1, 3.965×10¹¹ s^-1, f(α)=(1-α), respectively.