Abstract: Based on site classification to the studying area, the paper deeply analyses the relation between tree roots and site conditions and the influence of roots to tree growth as well as the function of soil and water conservation of tree roots. The results show as follows; (1) When the depth of soil layer on the water table is shallower than 1 meter, the tree’ s roots are difficult to form their root systems following their genetic characters, especially Robinia pseudoacacia, its main vertical roots will branch or turn direction of growth on the water table, and perhaps die of water drowning. So that the root system shows a horizontal form. When the soil depth is thicker than 1. 5 meters, the vertical root will grow well and deeply, generally, the depth is 1. 25-1. 5 meter. (2) When the obstacle layer of soil appears in the upper soil, ftobmia Pseudoacacia’ s roots are difficult to pass through and all of the roots gather in the upper soil. Because the roots of Metasiquoia glyptostroboides have stronger ability to pass through the tighter soil layer, there are not only some bigger vertical roots in the layer, but also many thinner roots under the obstacle layer, their roots are distributed in different soil layers. (3) The quantity of roots, especially, the roots which are thinner in diameter than 2 mili- meters, decreases with the increase of soil salt content and with the decrease of soil organism, Nitrogen and phosphorus content. (4) The growth quantity of trees and capability of prevention of soil erosion is increases with the increase of root quantity. The paper also presents some important suggestions which will have a great significance to the correct selection of afforestation tree species in the construction of protective forestry along the coast belt.