Recently, fused rings compounds have attracted an increasing focus since their outstanding features like high-energy, insensitiveness, high thermal stability, and so on. In order to gain the in-depth understandings of thermal decomposition behavior of them, the thermal decomposition process of TATOT was systematically studied through a series of means, including TG, DSC, TG-DSC-FTIR-MS, condensed-phase thermolysis in ARC and the tubular furnace assisted by SEM, FTIR, and XPS. The results revealed that pyrolysis process of TATOT was most likely to begin with a ring-opening reaction of the fused ring. The detailed mechanism and kinetic model of TATOT were proposed. Two-step consecutive mechanism A→B→C, which was described by f(α)=(1-α)^0.868 [1 + 1.364α^1.177] and f(α)=(1-α)^1.613, represented the kinetic model for TATOT pyrolysis the first stage. The kinetic model for TATOT pyrolysis the second stage was described by three-step consecutive mechanism C→D→E→F, where the first two steps were described by f(α)=(1-α)^3.570 and f(α)=(1-α)^1.888, and the last step was described by autocatalytic reaction model (KS). The typical safety indicators TMR_ad of TATOT, such as TD24 and TD8, were calculated to be 182.71 °C and 186.32 °C, respectively.