Wood carbon (WC)-derived thick electrode design has recently received increasing interest because of its high energy density at the device level. Herein, a facile, low-cost, and efficient strategy by surface engineering to synthesize ultrathick electrodes of quasi-solid-state symmetric supercapacitors (SSCs) based on activated wood-carbon (AWC) monoliths is presented. The AWC as a freestanding ultrathick electrode shows an impressive areal capacitance of 6.85 F cm-2 at 1 mA cm-2 and 4.55 F cm-2 at 20 mA cm-2. Furthermore, a quasi-solid-state SSC assembled by two identical AWC monoliths delivers an excellent energy density of 0.23 mW h cm-2 (4.59 W h kg-1 and 0.77 W h L-1) at 500 mW cm-2 (9.9 mW kg-1 and 2500 W L-1) while maintaining a capacitance retention of 86% after 10 000 cycles. The remarkable electrochemical performance is associated with the structural integrity of natural wood, the introduction of oxygen-containing functional groups, and the ultrathick electrode design, which significantly enhance electroactive material loading and device integration.