To develop bamboo-based products by weaving, winding and molding for promoting "bamboo as a substitute for plastic", it is urgent to fully understand the underlying mechanisms of flexible deformation and flexural toughness of bamboo at multiple scales. In this study, we quantified the micro deformation and strain of bamboo cells at different curvatures by in situ high-resolution field emission scanning electron microscopy (FE-SEM) and X-ray micro-computed tomography (Micro-CT) to analyze its flexible deformation at cellular and nano scales. The toughening mechanism embodied in fracture morphology was quantitatively calculated using mathematical models at macro and cell-wall scales. Results showed that bamboo possessed both excellent flexible deformation and flexural toughness in the longitudinal direction with the variation of fiber contents in the radial direction of bamboo culm. Flexible deformation of bamboo was achieved through synergistic cellular deformation between tensile sclerenchyma fibers (SFs) and compressible parenchyma cells (PCs) adapted to external stress distribution. SFs with greater tensile strain (0.027) bore the load even after the PCs broke (0.019), while the PCs in the compression layer acted as a buffer through plastic deformation. Meanwhile, the cell-wall sublayer wound by microfibrils at different angles, demonstrated significantly different Young's modulus and fracture modes verified by a mathematical model. Multiwalled fiber pullout and crack deflection were the main toughening mechanisms in bamboo at macro- and cellular scales. Multiwalled fibers pullout energy contributed the most to fracture energy dissipation, accounting for 46.0∼77.5%, whereas the weak intercellular interface effectively promoted crack deflection, reduced the local stress and delayed bamboo failure.