This study selected the two common tree species of Pinus tabulaeformis and Acer truncatum in North China. The objective was to investigate the root system tensile strength, the overall root strength of root-soil composite and three root principal component contents (cellulose content, hemicellulose content and lignin content) of the two tree species for root diameters from 0.2 to 6 mm. The results showed that with the increasing root diameter, plant roots got a higher hemicellulose content and cellulose content and a lower lignin content, followed by decreased root tensile strength. Plant root tensile strength had positive correlations with cellulose content and lignin content and, negative correlation with hemicellulose content. Therefore, beyond the root structure influence, the cellulose content and lignin content of plant roots are the direct causes of differences between the soil-binding capacity.