The quantitative relationships between the ecosystem factors and the contents of megastigmatrienones were studied by partial correlation analysis, path analysis and stepwise regression analysis methods. The experiments were conducted in 9 tobacco-growing areas distributed at Yanbian, Xichang, Dechang, Huili, Huidong, Ningnan, Puge, Mianning and Yuexi counties (cities) of Liangshan region, and the soil samples were taken from the representative land pieces of each county (city) before fertilizing the lands and before transplanting tobacco in order to avoid the raining season. The meteorological data used, such as temperature, rainfall and sunlight illumination were from the Sichuan Meteorological Administration. Eighty-one tobacco samples, including three grades of official tobacco standard GB2635-92, lugs(X2F)、cutters(C3F) and leaf (B2F) from the main strains (Honghua Dajinyuan and Yunyan 87) of each county (city), were taken with each sample weighing 5.0 kg. The standard methods in literatures were used to measure pH values, organic contents, contents of hydrolytic nitrogen, contents of all phosphorus elements, contents of quick-acting phosphorus, contents of quick-acting potassium, contents of all potassium elements in soils and contents of megastigmatrienones in flue-cured tobacco leaves. The results indicated that the ecosystem factors of the monthly mean temperature, the monthly mean amount of rainfall, the Accumulated temperature (≥20℃), the soil organic content, hydrolytic nitrogen content, quick-acting potassium content and the pH value, played determinant rules in the contents of megastigmatrienones in flue-cured tobacco leaves. These factors were not independent in their effects, and controlled about 98.02% of the changes of megastigmatrienones content in flue-cured tobacco leaves. The key ecosystem factor controlling the contents of megastigmatrienones in flue-cured tobacco leaves was the monthly mean amount of rainfall, and its effect was negative. Both the organic content in soil and the monthly mean temperature had positive effects on the contents of megastigmatrienones. The organic content was the main factor promoting the content of megastigmatrienones while the accumulated temperature (≥20℃) was the main factor limiting the contents of megastigmatrienones in flue-cured tobacco leaves.