Abstract: In order to understand the content of exchangeable Mg in soil and Mg in flue-cured tobacco leaves, the soil and tobacco samples were collected and studied by point to point approach in Longnan. The results showed that: the moderate exchangeable Mg in soil was reached to 72.4%, the abundant content was reached to 27.6%, the overall content ranged from moderate to abundant, and the exchangeable Mg content increased gradually from northwest and southeast to the middle. Despite the exchangeable Mg in soil was rich, Mg in tobacco leaves was insufficient, of all the tobacco leave samples, 2.74% of the samples lacked Mg significantly, 83.56% of them lacked Mg slightly, and only 13.70% of them had suitable Mg content, so the overall content was comparatively low. The Mg in tobacco leaves was significantly positive correlated to exchangeable Mg and organic matter, and was extremely significantly negative correlated to the ratio of exchangeable Ca to Mg. The higher ratio of exchangeable Ca to Mg, and the antagonism between exchangeable Ca and Mg might be an important reason of low Mg content in leaves. To balance fertilization, coordinate the ratio of exchangeable Ca and Mg, increase the application amount of organic fertilizer appropriately and have top dressing of Mg fertilizer are important ways to improve the Mg content in flue-cured tobacco leaves in Longnan.