Abstract: To study the relationship between soil organic matter (SOM) contents and environmental factors, 1 698 soil samples were collected from southeast Chongqing tobacco-growing areas, and the SOM contents in these samples were statistically analyzed. A Classification and Regression Tree (CART) model was developed to analyze the effects of 11 environmental factors such as terrain, climate and parent materials on SOM contents. The results showed that:1) SOM contents presented a normal distribution with the mean of 25.82 g/kg, and possessed moderate variability with a variation coefficient of 30.00%. A total of 56.01% of the area was suitable for planting tobacco. 2) Different correlations existed between SOM contents and different environmental factors. SOM contents significantly positively correlated to altitude, height above riverway, terrain humidity index and annual average precipitation, while significantly negatively correlated to slope gradient, slope-length factor, flow path length and annual average temperature. The SOM contents in southeast slope area were the highest. 3) The CART model could explain 64.80% of spatial variability in SOM contents. The importance of major environmental factors on SOM contents was in the order of:altitude > annual average precipitation > annual average temperature > parent material.