A composite fiber nitrocellulose/glycidyl azide polymer/nanometer 1,3,5-triamino-2,4,6-trinitrobenzene(NC/GAP/nano-TATB) with three-dimensional structure was prepared by electrospinning method. Differential scanning calorimeter(DSC) and online thermal-infrared spectrometry(TG-IR) measurement were conducted to probe the low temperature thermochemical properties of the composite fiber. Result indicates that there is only one exothermic peak existing in its DSC curve, which means that NC, GAP, and nano-TATB decomposed simultaneously rather than decomposed individually. The activation energy (E_a) of NC/GAP/nano-TATB (208.1 kJ·mol^-1) is lower than nano-TATB (228.9 kJ·mol^-1), and the rate constant (k) of NC/GAP/nano-TATB (1.70 s^-1) is higher than nano-TATB (0.92 s^-1). The composite fiber is easier to be activated and will decompose faster than nano-TATB. The main products for thermal decomposition of NC/GAP/nano-TATB include CO₂, N₂O, NO, CO, NO₂ and H₂O, meanwhile, fragments like ─CH─, ─CH₂O, and C─O─C were also detected. Moreover, the energetic performance and sensitivity of the composite fiber have been detailedly evaluated and compared with that of NC/GAP and nano-TATB. Combustion temperature (T_c) of NC/GAP/nano-TATB is up to 1583 ℃ and the addition of nano-TATB is favorable to the reduction of impact sensitivity.