Nano-TATB (1, 3, 5-triamino-2, 4, 6-trinitrobenzene) is an insensitive high explosive (IHE), some impurities in its preparation process were produced. In this work, the structures of the impurities were studied by liquid-state ¹³C nuclear magnetic resonance spectroscopy, X-ray photoelectron spectroscopy and theoretical simulation method. Results show that the impurity is the proton compound of TATB molecule and it is named as 1, 3, 5-triamino-2, 4, 6-trinitro-2, 5-cyclohexadiene bisulfate. According to the quantum chemical method, the possible formation mechanism of this kind of impurity is the protonation of TATB molecule by H atom in concentrated H₂SO₄ during the dissolution process.