<p>To elucidate the influence of wood fiber size distribution and resin content on the elastic constitutive behavior of high-density fiberboard (HDF), a transversely isotropic constitutive model was used based on the mat-forming and hot-pressing processes and the characteristic density profile of HDF. Displacement fields under three-point bending were obtained experimentally using digital image correlation (DIC), analytically from Timoshenko beam theory, and numerically through finite element model updating (FEMU). These displacement fields were integrated to identify five independent elastic constitutive parameters— transverse elastic modulus, longitudinal elastic modulus, longitudinal shear modulus, transverse Poisson's ratio, and longitudinal Poisson's ratio—by means of nonlinear least-squares optimization. The identified parameters were compared among HDF specimens manufactured with different wood fiber size distributions and resin contents. Furthermore, the microscopic fracture morphologies of bending failure surfaces were analyzed to clarify the underlying mechanisms by which fiber size distribution and resin content affect the mechanical performance of HDF. The results demonstrate that the mechanical properties of HDF exhibit pronounced directional dependence, which is governed by fiber interweaving, fiber–resin interfacial bonding, and internal pore defects. Fine fibers fill internal pores and reduce structural defects, while medium and long fibers form the primary load-bearing framework. Excessive coarse fibers hinder fiber interlocking and promote pore formation, degrading mechanical performance. Increasing resin content enhances interfacial bonding, thereby improving static bending strength and elastic moduli. This study provides a methodological reference for characterizing the parameters of wood-based materials such as HDF.</p>