Phase change material (PCM) with thermal energy storage capacity and automatic temperature regulation hold tremendous potential for construction energy conservation. However, conventional PCMs often exhibits liquid leakage, volume change and insufficient mechanical property, which are prone to load-bearing accidents, hindering their application as an engineering building material. Herein, inspired by the oriented “brick-and-mortar” structure in bone, we report a facile strategy to develop a form-stable, ultrastrong and highly energy-stored composite wood-based PCM (PWPCM) via in-situ polymerization of polyurethane-based PCM using hierarchically well-aligned wood as bio-template. PWPCM showed high tensile strength of 81.9 MPa and elastic modulus of 10.2 GPa along the longitudinal direction due to the well-aligned cellulose nanofibrils arrays and the “brick-and-mortar” structure based on strong intermolecular bonding between cellulose and PCM chains, as confirmed by molecular dynamic simulations. PWPCM was 73.4 times stronger than pure PCM, and 4∼40 times as high as most conventional PCMs ever reported, representing one of the strongest composite PCMs. Benefited from the high-loading PCM, PWPCM showed wide phase transition temperature range and high enthalpy of 116.1 J/g, together with outstanding temperature regulation and shape-stability. This work provides a promising strategy for fabricating low-cost, eco-friendly, strong wood-based PCM for energy-saving construction and intelligent agriculture.