Abstract: Light intensity measurements on 8 azimuth at the secondary forest canopy gap were conducted in Xishuangbanna, Yunnan, in Jan. Apr. Jul. and Oct. 1999.On the basis of this measurement, spatial and temporal variations of light intensity in the secondary forest canopy gap were discussed. The results indicated that: because of the influence of condition of foggy environment and sun altitude and the tree, spatial and temporal variations of light intensity on the forest gap are significant in different time in different season, that is the phenomena about peak value displacing and spatial asymmetry, and the source of maximum values of light intensity do not present to the center, but to the other side, and that it displays shifting along with time. Temporal diversity and spatial Asymmetry is the characteristic of distribution of light intensity, it makes thermodynamic regime different on different sites, and results in differences on caloric transmitting and caloric storing in the gap, finally the different thermodynamic effect is generated.