Windward lateral roots play an important role in the resistance of trees overturning under typhoon, however, few studies focus on their behaviour under large deformation. Based on physical model tests and finite element simulations, the force-displacement curves of root elements under large displacements were obtained. Then the bilinear softening assumption, incorporating the Winkler foundation beam method was used. Effects of root diameter and length on the peak bending moment and rotational stiffness were investigated. The results showed that when the root was smaller than the critical length, the peak bending moment raised with the increase of diameter and length, and it was more sensitive to the diameter. Additionally, the overall rotational stiffness and initial rotational stiffness both increased with the increase of diameter, however the overall rotational stiffness decreased with the length, while the initial rotational stiffness basically unchanged. When the root was longer than the critical length, the moment-rotation curve was no longer changed in general, so the above indexes were therefore unchanged. The critical length was fitted using the root diameter and modulus of elasticity, which can meet the estimation of the critical length of thick roots for engineering purpose.