<p>Microwave treatment has shown potential to improve the hygroscopic stability of wood via cross-scale interactions between chemical composition and pore structure. This study investigated the hygroscopic behavior of microwave-treated <i>Cunninghamia lanceolata</i> earlywood and latewood, revealing different responses driven by changes in chemical composition and pore structure synergy. The experimental results showed that microwave treatment reduced the equilibrium moisture content across a wide range of humidity, with the reduction in earlywood being pronounced than that in latewood. At the molecular level, modification of chemical composition, especially those resulting from reduced hemicellulose and hydroxyl accessibility, played a dominant role in limiting the moisture absorption capacity. At the cellular structural level, microwave-induced vapor pressure reshaped the micro-mesoporous network structure, including separation of rimmed pits and cell wall microcracks, further limiting the water retention capacity of wood. The high sensitivity of earlywood to microwave treatment stems from its inherent structural chemistry, which exacerbates chemical degradation and structural damage. These findings establish a coupling mechanism between structural and compositional changes, whereby chemical absorption site loss supersedes pore structure evolution in controlling hygroscopicity, providing key insights for optimizing microwave parameters to improve hygroscopic stability of wood.</p>