1, 1-diamino-2, 2-dintroethylene (FOX-7) is a novel explosive of high energy and low sensibility. In order to study the effect of temperature changes on the molecular structural characteristics of the explosive, its absorption spectra in the frequency range of 0.2–2.5 THz at a constant rate of heating from 298 K to 393 K are detected by terahertz time-domain spectroscopy (THz-TDS). Results show that a number of characteristic absorption peaks with different intensities appear at 1.59–2.13 THz when the temperature is 298 K, while the absorption spectra change with the increase of temperature of the explosive sample; a new characteristic absorption peak located at 1.12 THz appears at 384 K, and its absorption peak intensity gradually increases, but disappears when the temperature drops to 298 K. The absorption spectra of FOX-7 molecular crystal at 298 and 393 K within the 0.2–2.5 THz region based on density functional theory (DFT) are also simulated by using Materials Studio 6.0 software in this article, and the simulated results agree well with the experimental data. In addition, the vibrational modes of the characteristic peaks of two kinds of crystalline in the experimental absorption spectra are analyzed and identified, showing that the formation of the characteristic absorption peaks is closely related to the molecular vibration, and the molecular structure may change under the influence of temperature, and the tautomeric polymorphism of the crystalline has different vibrational modes. This article indicates that the process of phase transformation of FOX-7 starts from 384 K, and this process is reversible; the characteristic absorption peak at 1.12 THz is composed of two kinds of vibrations (the swinging and torsional vibrations of the nitro and amido groups).