Abstract: Microclimatic measurements were conducted in the canopy gap of tropical secondary forest in Xishuangbanna in fog-cool and dry-hot season.The daytime thermal effect of different thermal active surface in the canopy gap was discussed,and the variations of trunk surface temperature near gap edge and of surface temperature on the gap were analyzed.The result shows that the woody-wall surface is new thermal active surface on the vicinage of canopy gap,with the exception of the forest canopy surface, soil surface in the forest gap,and soil surface of the interior. Because of the influence of season,situs and time,the thermal effect of different thermal active surface of gap was significant different. Being subject to fog, the soil surface on the center of canopy gap is an important thermal active surface in the morning;while in the midday and afternoon,because of the influence of incident radiation, the woody wall and soil surface of edge on the east and the woody wall surface of edge on the north are the key thermal active surface of the canopy gap. In the midday,the thermal active was salient on the surface of gap,and in the afternoon,it was salient on the woody wall. The thermal variations of canopy gap of forest are controlled by the thermal characteristics of different thermal active surfaces and by their interaction,affecting the growth of plants.