Wood aerogel is a new type of cellulosic aerogel , referring to mesoporous wood with a partially preserved hierarchical structure and nanofibril networks filled lumen. It outperforms common cellulosic aerogels due to facile top-down fabrication and good mechanical strength . This work shows a green, benign and scalable approach for wood aerogel synthesis using one-step NaOH (10 wt%) treatment under room temperature. Highly mesoporous wood aerogels with specific surface area of 202 m2/g, and yield strength of 4.3 MPa were obtained. The generation of mesoporosity and the building of fibril networks in the lumen were studied. NaOH swells the cell wall, partially extracts cell wall components, and disrupts the bonding between fibrils when it encounters wood. Disintegrated fibrils and extracted hemicelluloses then reassembled into fibril networks occupying the lumen.   The technology developed for wood aerogel synthesis and the mechanism understanding of wood aerogel formation pave the way for green cell wall nanoengineering towards advanced materials design.