In order to improve the fire extinguishing efficiency of forest fires and reduce the damage to forest resources and ecological environment, a three-phase class A foam extinguishing agent consisting of class A foam extinguishing agent with nano-SiO₂ particles was prepared. The effects of nano-SiO₂ mass fraction on foam stability and fire extinguishing efficacy were investigated by using the double injector method and the wood stack fire extinguishing test platform. The results show that the nano-SiO₂ significantly enhanced the foam stability and delaye the foam precipitation and foam coarsening; the three-phase class A foam extinguishing agent extinguishe the flame more quickly and efficiently under the same driving pressure without rekindling and at a lower dosage than the traditional class A foam extinguishing agent; and the three-phase class A foam extinguishing agent with 1.5% SiO₂ show the best extinguishing efficacy. The three-phase A-type foam extinguishing agent significantly improves the fire extinguishing efficiency, reduces the consumption of extinguishing agent, and has low cost, which makes it suitable for large-scale forest fires. The study provides a new solution for the efficient extinguishing of forest fires and has a wide range of practical applications.