The Daxing 'an Mountains is one of the frequent occurrence areas of forest fires in China, with the average annual burned area ranking first in the country. In recent years, under the influence of extreme climate phenomena, both the occurrence frequency and hazard level of forest subsurface fires have both increased. Moreover, subsurface fires have the characteristics of concealment, randomness and unpredictability. Therefore, clarifying the main driving conditions that affect the risk factors of forest subsurface fires is of great significance for the protection of forest resources and the safety of fire-fighting personnel in the Daxing 'an Mountains. This study investigated the risk factors of forest subsurface fires in the Daxing 'an Mountains through field investigations and outdoor simulated point burning experiments. The results indicated that during subsurface fire combustion, it first spread from one or several points in the near surface layer to the surrounding area, and the temperature also increased accordingly. After removing the heating plate, some areas experienced a brief decrease in temperature before rising again and maintaining it for a period of time, before gradually decreasing to room temperature and extinguishing. The concentration of smoke gases released during the initial concentration stage of subsurface fire combustion was relatively high and exhibits significant fluctuations. Afterwards, the release concentration was relatively more stable. After removing the heating plate, the release concentrations of individual smoke gases components were increased to varying degrees. There were significant differences (P < 0.05) in the maximum combustion temperature of subsurface fire under different fuel types and moisture contents. The combustion temperature under the conditions of Tatoudianzi and a moisture content of 25% was generally lower. The combustion temperature of subsurface fire was higher under conditions of moisture content of 20% and 35%. There were significant differences (P < 0.05) in the CO₂ concentration, CO concentration, PM_2.5 concentration, PM₄ concentration, PM₁₀ concentration, and TSP concentration released by the combustion of subsurface fire with different fuel types. There were also significant differences (P < 0.05) in the CO concentration, PM₁ concentration, PM_2.5 concentration, PM₄ concentration, PM₁₀ concentration, and TSP concentration released by the combustion of subsurface fire under different moisture contents. Relatively speaking, the smoke gases release concentration is higher under the different moisture contents of poplar-birch mixed forest and under the condition of 35% moisture content in different fuel types. The relevant research results can provide theoretical basis and technical support for the prevention and extinguishing of subsurface fires in the region.