The content and the distribution of soil available magnesium of 31 provinces in China were studied by soil systematic approach with 17790 selected soil samples. The results indicated that the content of soil available Mg was from 1.2 mg/L to 4468.9 mg/L, the average content was 320.6 mg/L. According to the evaluate index of soil systematic approach, about 8% of the selected soil was seriously Mg deficient, 13% was potentially deficient, 33% was in the middle level, 34% was sufficient and 12% was excessive. There were about 54% of soils that should supply magnesium fertilizer and the soils with low available Mg was mostly distributed in south Changjiang-river, such as Fujian, Jiangxi, Guangdong, Guangxi, Guizhou, Hunan, Hubei. It estimated that the annual need of magnesium fertilizer would be about 9840 KT in China.