Abstract: In order to explore the effects of soil water stable aggregates and soil mechanical composition on soil physical and nutritional characteristics in typical tobacco-planting soil of Southwest China, soil samples from five areas were collected in Baoshan, Yunnan Province. The indicators of water stable aggregates, soil mechanical composition, soil physical and nutritional characteristics were analyzed. The relationships of soil structure composition with soil physical and nutrient characteristics were discussed. The results showed that the content of soil water stable aggregate in different areas was the highest with the particle size of >5 mm, the lowest with the particle size of 0.5-0.25 mm. The mechanical composition of soil was the highest with the coarse silt, and the lowest with the medium sand. However, the mechanical composition of soil in different areas was significantly different, mainly between the coarse sand, fine sand, fine silt and clay. In addition, there were significant differences in soil physical and nutritional characteristics among the five typical tobacco planting areas, in which Tengchong area had better soil physical and nutritional characteristics. Correlation analysis and redundancy analysis showed that there was a significant correlation between the composition of soil structure composition (water stable aggregates and soil mechanical composition) and soil physical and nutrient characteristics. The composition of soil water stable aggregates had a significant impact on soil nutrient characteristics, among which <0.25 mm particle size aggregates (F=5.4, p=0.03) were the key factors leading to nutrient differentiation in different regions. The mechanical composition of soil had a significant effect on the physical characteristics of soil, among which silt (F=28.9, p=0.006) and medium sand (F=12.5, p=0.048) were the key factors that cause the difference of physical properties of soil in different regions. In conclusion, <0.25 mm aggregate, silt and medium sand play an important role in regulating soil physical and nutrient characteristics in typical tobacco growing areas in Southwest China.