Self-hardening thermoplastic foams (STFs), as an innovative technology mainly synthesized using 1 wt. % auxiliary agent (organic acid and polycaprolactone) and 99 wt. % foaming solution (polyethylene oxide, sodium silicate solution, sodium dodecyl sulfate, and water), were proposed to inhibit the self-oxidation of coal. When gas was injected into the synthetic solution using the self-made device with gas as the single power source, due to the synergistic action of the surfactant and polyethylene oxide, an aqueous foam with an excellent wetting ability was initially formed, followed by organic acids gradually reacting with sodium silicate, with the sol-gel foam and STF obtained after 180 and 220 s, respectively. The thermal characteristics of STF inhibiting the self-heating of coal were analyzed by differential scanning calorimetry, X-ray photoelectron spectroscopy, electron paramagnetic resonance, and Fourier transform infrared spectroscopy. After the STF absorbed the heat generated by coal self-heating, it was fused into a colloid in the range of 42-93 degrees C. The colloid could seal coal gaps and cracks as a thin whole colloid layer from oxygen ingress, thus, the initial reactions of carbon free radicals and methyne with oxygen were controlled. The colloid could destroy the active functional groups, leading to an obvious decrease in the relative content of C-O. The colloid could keep the allowed water to maintain the moisture of the coal body, resulting in a peroxygen content decrease. The colloid could prevent cycloalkanes and bridge bond breakage from generating new alkyl groups. The colloid also provided hydrogen ions that could destroy peroxy, alkoxy, and hydroxyl radicals. Therefore, the STF could cut off the free radical chain reaction of coal and prevent its further oxidation.