In order to explore the reason for the oak seedling’s high mortality in the mixed forest, the author used the growth of plant root as the starting point to find the variation of the roots and understand the influence of root morphology to the growth of seedling. The author analyzed the seedling’s growth and root morphology of 3 age classes (1, 2, 3-year-old) Quercus liaotungensis in a Pinus tabulaeformis+Q. liaotungensis mixed forest stand on loess plateau. The results showed that: with increasing age the root-shoot ratio, seedling root surface area and length were increased, which indicated that the underground parts of seedlings needed more photosynthetic products. Root morphology studies had shown that with increasing age, the changes of root composition led to fine roots become thinner, which would cause the belowground biomass allocation increased in the 2, 3-year-old seedlings. Thin fine root and surface area decreases would affect the root stability and absorption. Correlation analysis showed that there was a correlation between root morphology and aboveground biomass, but not linear, which indicated that there had a complex relationship between root morphology and the biomass. A further application of the control experiments with continuous measurement would help people to find out a deep relationship between them. The root morphology was changed with the seedling’s growth, which affected the root stability and biomass allocation immediately. However, such effects were complex. So people needed a further research and exploration.to find out how the root changes influence the seedling’s growth and the ability of resource competition under the high canopy mixed forest.