The three-dimensional nanostructured and degradable polyhemiaminal aerogel (PHA) was used as template to induce the in situ crystallization of 1,1-diamino-2,2-dinitroethylene (FOX-7) by evaporation crystallization method. The FOX-7 with three-dimensional nanostructure (nano-FOX-7) was prepared after the degradation of the PHA in dilute sulfuric acid (10%). The purity of nano-FOX-7 was determined to be 99% by high performance liquid chromatography (HPLC), indicating that the template was almost completely removed. The morphology, phase, structure and thermal properties of the samples were characterized. Detemplated nano-FOX-7 preserved three-dimensional nano structure of PHA. The average size of nano-FOX-7 crystal particle was 83.68 nm. Owing to the special micro-nano structure, the peak temperature of phase transition and first thermal decomposition temperature for nano-FOX-7 was delayed by 11.3 °C and 21.3 °C, respectively. Compared with raw FOX-7, the thermal decomposition of nano-FOX-7 almost completely took at exothermal profile with peak of 291.0 °C. The enthalpy of the thermal decomposition increased from 1309 J·g^-1 (raw FOX-7) to 1421 J·g^-1 (nano-FOX-7), together with 31.6% increase of the apparent activation energy (from 408.80 kJ·mol^-1 to 537.42 kJ·mol^-1), which explained the remarkable improvement of the energy release efficiency and thermal stability for the nanostructure FOX-7.