<p>Plant fibres are emerging as sustainable composite reinforcements. Compared to man-made fibres, plant fibre reinforced composites (PFRCs) possess intrinsic microstructures, i.e., hierarchical micro-structure of plant fibres and twisted meso-structure of plant yarns, leading to their complicated mechanical response on the macro-scale. Existing theories, assuming homogeneous fibres, cannot accurately describe the unique coupling microstructure-mechanism of PFRCs. Therefore, in this study, a multi-scale analysis method incorporating inherent microstructures of flax fibre reinforced composites (FFRCs) was proposed, focusing on the effect of the evolution of microstructures, especially the microfibril angle (MFA) in micro-scale and twist angle in meso-scale, on the macroscopic tensile behaviors of FFRCs. This refined numerical modeling approach spanned a hierarchical constitutive model upon finite transformation that elucidated the nonlinear mechanism induced by MFA evolution in micro-scale, a fine model with twisted structure of impregnated flax yarns that tracked the twist angle time-varying behaviors in meso-scale, and a precise model of FFRCs coupling the evolution of MFA and twist angle in macro-scale. The proposed model was compared with the models neglecting the microstructural evolution and also with the experimental results. It was shown that the evolution of MFA and twist angle considerably affected the nonlinear tensile behaviors of FFRCs. A significant agreement between the experimental and numerical results was achieved, considering the synergistic effect of the microstructural evolution during the tension of FFRCs. These findings address the dominating microstructure-induced mechanism in PFRCs, enabling better utilization of PFRC structures in the future and deeper the understanding of mechanics mechanisms for bio-inspired materials.</p>