In order to quantitatively evaluate the effect of different root contents on the shear strength of the root-soil composite, the distribution characteristics of the root area ratio of the Bischofia javanica with the increase of buried depth were analyzed. An improved large-box direct shear experiment was carried out on the remolded vertical root-soil composite with seven different root area ratios and four different normal stress levels. The changing rules of the soil cohesion and internal friction angle under different root area ratios were studied. The results showed that the shear strength of the root-soil composite of the Bischofia javanica was increased by 1%~25% compared with the soil without roots. Under the same root area ratio, the shear strength of the root-soil composite increased when the normal stress increased, but with a decreased growth rate. With the increases of the root area ratio, the shear strength of the root-soil composite increased first and then decreased. There was an optimal root content to maximize the shear strength of the root-soil composite. The peak shear strengths under four different normal stress levels are 100, 121, 136, 142 kPa respectively, and the corresponding root area ratios were all 0.20%. When the root content exceeded the optimal value, the attenuation rate of the shear strength was related to the normal stress. The larger the normal stress, the smaller the attenuation. The effect of the root area ratio on enhancing the shear strength of root-soil composite was mainly reflected in the soil cohesion, while its influence on the internal friction angle was small. The results of this study could provide some theoretical bases for stabilizing slopes using Bischofia javanica roots and have positive significance for further understanding the interaction between the arbor roots and soil.