This paper mainly discussed the monthly dynamic changes of the nutrient content of root and soils nutrient in the model of Triploid populus tomentosa and Lolium multiflorum Lam in cultivating lands to forest. Based on detailed investigating forest lands,analysing samples and processing datum,the conclusions can be drawn:In the model,the number of Lolium multiflorum Lams root absorbing N element was 0.3950.408 g/kg,and it was higher than that of tree root about 0.1730.369 g/kg,so the tree was the main factor that caused the change of the living being absorbed the N element. But the absorption to efficacious P and N was just in the opposite. The number of tree roots absorption was 2.4863.486 g/kg, and that of grass root was 4.5726.092 g/kg. On the other hand,the change of content of tree roots K was relatively big,but the grass roots change was small,and soils K usually was supplied abundantly. With theCa content in the forest land increasing, the roots of Triploid populus tomentosa increased,and it was 4.6519.764 g/kg,but the grass roots absorption was just opposite. The Mg demand of tree root and grass root respectively was 10.54710.744 g/kg and 10.36510.610 g/kg. It was similar not only on the trends of seasonal variation,but also at the number of demand,so the characteristic of utilizing and absorbing should be pay attention to in applying Mg fertilizer.