3 506 samples of flue-cured tobacco leaves were collected from Qujing of Yunan Province and their Mg contents were analyzed, then decision tree analysis and regression analysis were used to study the influential factors. The results showed that Mg contents ranged from 0.36 g/kg to 17.70 g/kg with a mean of 4.10 g/kg in flue-cured tobacco leaves, and Mg contents were lower than 4.00 g/kg in 46.36% of the samples. Altitude, soil type, texture, organic matter, available potassium, available magnesium, and available calcium affected Mg content of flue-cured tobacco. Mg content of flue-cured tobacco decreased with the increase of altitude and the fining of soil texture. It was significant difference in different soil types, the highest (4.98 g/kg) in purple soil and the lowest in red soil (3.82 g/kg). Mg content decreased with the increase of SOM, gradually increased with the increase of soil available Mg and available K, and increased first and then decreased with the increase of soil available Ca. Mg content reached the highest while soil available Ca was about 2 g/kg. And Mg content decreased with the increase of soil Ca/Mg an K/Mg ratios.