A new compound of mono-ammonium trinitrophloroglucinolate ［NH₄(H₂TNPG),TNPG=Trinitrophloroglucinol］ was synthesized and characterized by elemental analysis and Fourier transform infrared (FT-IR) spectrum. Its crystal structure was determined by single crystal X-ray diffraction analysis. The crystal belongs to monoclinic, C2/c space group, a=9.307(2)Å, b=21.144(6)Å, c=9.797(2)Å, β=99.56(3)°, V=1901.0(8)ų and Z=8. The compound is an ionic compound consisting of a cation NH_{4⁺ and an anion C6H2N3O9^-, and it is the mono-substituted salt of TNPG. Since the existence of complicated hydrogen bond networks and electrostatic attraction between NH4⁺ cation and C6H2N3O9^- anion, the compound possesses better stability and low sensitivity. The thermal analysis of the compound was studied by using differential scanning calorimetry (DSC), and thermogravimetry thermogravimetry-derivative analysis (TG-DTG). Under nitrogen atmosphere with the heating rate of 10 ℃·min^-1, the thermal decomposition of the compound show only one intense exothermic decomposition process at 219.2-234.3 ℃ on the DSC curve, and the decomposition products are nearly all gaseous products. Sensitivity Test revealed that the title compound is very insensitive to external stimuli.}