The damage evolution and structural failure in trees and other plants are mainly originated from the plastic yielding in the wood cell wall at the microstructural level, which consists of cellulose fibrils embedded in a polymer matrix of hemicellulose and either lignin or pectin. Understanding the mechanical behavior of wood cell wall at the plastic regime is critical to the investigation of the fracture characteristics of trees at macro-scale. In this research work, the wood cell wall, which consists of cellulose fibrils, hemicellulose chains and lignin macromolecules, is modeled at the mesoscale to investigate the mechanical responses under deformation. By examining the force-strain relationship, the mechanical behaviors of the wood cell wall at the plastic yielding range are obtained, which are initiated from the slippage between the fibrils and polymer matrix. The simulation results are compared with experimental measurements and theoretical predictions to provide a bottom-up description of micromechanics of the wood cell wall, and to explain the damage evolution and structural failure occurred at the larger scales. The wood cell wall investigated here can be applied to the construction of wood hierarchical structure as a basic unit and adapted to the studies of different natural materials.