As a bridge of material exchange and nutrient cycle between plant and soil, root system has been a research hot spot in ecology. The aim of this study was to explore the potential ecological relationship between the roots of two species. We carried out the research in the growing season in July 2017. The root system was collected in 11 years nitrogen (N) control simulation test sample of Betula platyphylla & Populus davidiana natural secondary forest in Changbai Mountains. The root sequence method was used to reveal responses of the root morphology and anatomical structure to different gradients of N addition. In the study, three N addition gradients were set, which were ambient environment (CK, 0.0 g N m^-2 a^-1), low nitrogen addition (T_L, 2.5 g N m^-2 a^-1), and high nitrogen addition (T_H, 5.0 g N m^-2 a^-1), respectively. The results were as follows. 1) The T_L significantly inhibited growth of the cortical thickness of the first three levels of roots of Betula platyphylla and Populus davidiana. Betula platyphylla reduced cortical thickness by increasing cortical cell diameter (the primary root increased by 72.77%, the secondary root increased by 53.22%, and the third level root increased by 39.96%). While Populus davidiana reduced cortical thickness mainly by decreasing cortical cell diameter (the primary root and the secondary root decreased by 40.80% and 28.17%, respectively). 2) The T_H significantly inhibited the growth of the first three levels of roots of Populus davidiana, which was inhibited mainly by increasing the thickness of the cortex (the primary root increased by 68.78%, the secondary root increased by 50.81%, and the third level root increased by 88.53%) and by decreasing the cross sectional area of the catheter so as to affect growth. 3) By comparing the T_H with the T_L, fine roots diameter of Betula platyphylla showed growth inhibition by inhibiting the growth of the stele diameter (the primary root decreased by 17.61%, the secondary root decreased by 16.89%, and the third level root decreased by 20.62%). The results indicate that the fine roots of Betula platyphylla and Populus davidiana respond differently to N deposition of different concentrations in the same site conditions.