Building the 3-D model of individual tree and analyzing completely the spatial distribution of leaf area index (LAI) are of great significance to understand the process and prevention of soil erosion under the forest in the south of China. A case study was conducted in the experiment site of the joint Chinese-German-Swiss research project “BEF-China”, which was located in the hilly region of the eastern part of Jiangxi Province, China. Initially, the point cloud data of a 5-year-old Chinese fir tree was obtained by using terrestrial laser scanner (TLS), then the spatial distribution of LAI was extracted and the impact of crown structure, its spatial heterogeneity on soil erosion under the forest was analyzed. The results showed that TLS was a powerful tool for reconstructing the 3-D model of individual trees. An effective segmentation algorithm to extract the radial distribution of canopy elements was presented. The spatial distribution of LAI of the whole tree was uneven: LAI (ΔH=0.1m) ranged from 0.02 to 0.18 m²/m² in the vertical direction, while radially LAI values, close to the trunk, could reach 8 – 25 times of those on the crown edge. LAI in the downhill was greater than that in the uphill. LAI of the whole fir tree was 2.45 m²/m². In addition, it was found that TLS was sensitive to wind in our field measurement and the voxel size was a crucial factor in predicting LAI with the voxel-based method.