Forest underground fire occurs in the humus. It is a slow, flameless, low-temperature, persistent smoldering combustion maintained by the heat released in the whole combustion process. Therefore, the temperature of underground fire is an important indicator for studying the characteristics of underground fire behavior, and it is also an important basis for monitoring and fighting forest underground fires. Larix gmelinii plantation under five land classifications in Daxing'an Mountains was taken as the study object. Based on the indoor control burning experiment, we studied the temperature changes of smoldering combustion under different land classifications and particle sizes of humus. There was no significant difference between the highest temperatures of smoldering combustion of humus with different particle sizes (P>0.05), while the effects of different land classifications and their interactions on the maximum temperature of smoldering combustion were significant (P<0.05). There were significant differences in the maximum temperature of smoldering combustion between different land classifications under four humus particle sizes (P<0.05). The smoldering combustion temperature of Tatoudianzi could go up to 897.53℃, which was the highest of all humus particle sizes, and followed by meadow bog, slope mountains, flat mountains, and agricultural land, where the smoldering combustion temperatures of humus were lower. The surface temperature of humus burning in the same land classifications was relatively high, and the highest temperature could reach 618.83℃. The surface temperature of humus combustion decreased with the increase of combustion time. The relationship between them could be fitted by y=a×x^b, and the fitting degree was high (R²>0.9, P<0.01). The results can provide a scientific and effective theoretical basis for monitoring and fighting forest underground fires in this area.