Abstract: Based on historical fire data, in this study the underground fires occurred in Daxing'an Mountains in Heilongjiang Province are investigated, combining with plots setting up in the field, the characteristics of combustible moisture content and calorific value are measured, and the burning experiments are simulated to study the smoldering characteristics of underground fires in forest steppe. The results showed that the average fuel load in the fire area of underground forest was 37.3 t/ha, and the average proportion of herbaceous layer, semi-decomposed layer and humic layer in the forest underground fire area was 18.50%, 28.95% and 52.55%, respectively; the burning temperature of semi-decomposed matter and humus was 405, 525 ℃, respectively. The highest can reach 690 ℃. The temperature and persistence of humus smoldering was much higher than other combustibles. The flammability ranked: humus layer>semi-decomposed layer>herb layer; the water content of combustibles was inversely proportional with the temperature rise rate of combustibles; as the load of combustible materials increased, the heat released by combustion increased sharply, the time of the highest temperature began to gradually decrease. This study can provide data support for the research on fire prevention of forest steppe, and frontline staff.