Lightweight, highly strong and bio-based structural materials remain a long-lasting challenge. Here, inspired by nacre, a lightweight and high mechanical performance cellulosic material was fabricated via a facile and effective top-down approach and the resulting material has a high tensile strength of 149.21 MPa and toughness of 1.91 MJ/m³. More specifically, the natural balsawood (NW) was subjected to a simple chemical treatment, removing most lignin and partial hemicellulose, follow by freeze-drying, forming wood aerogel (WA). The delignification process produced many pores and exposed numerous aligned cellulose nanofibers. Afterwards, the WA absorbed a quantity of moisture and was directly densified to form above high-performance cellulosic material. Such treatment imitates highly ordered “brick-and-mortar” arrangement of nacre, in which water molecules plays the role of mortar and cellulose nanofibrils make the brick part. The lightweight and good mechanical properties make this material promising for new energy car, aerospace, etc. This paper also explains the strengthening mechanism for making biomimetic materials by water molecules-induced hydrogen bonding and will open a new path for designing high-performance bio-based structural materials.