Keeping outstanding electrochemical performance under high mass loading is critical to develop functional thick electrodes. Here, we report a high phosphorus-doped wood-derived carbon thick electrode for supercapacitor via phytic acid treatment, which can form hydrogen bonds with cellulose molecules in the wood. The content of phosphorus reaches up to 9.24 at% in carbonized wood with P-doping (CW-P-9.24), higher than most previously reported P-doped carbonaceous materials. CW-P-9.24 electrode (800 ?m, 17.17 mg cm-2) exhibits greatly improved electrochemical performance, especially in energy density and cyclic stability. Significantly, the symmetrical supercapacitor device exhibits high areal and specific capacitance of 4.7 F cm-2 and 206.5 F g-1 at 1.0 mA cm-2 with prominent retention of 90.5% through the long-term cycling at 20 mA cm-2 for 20,000 cycles, and 0.94 mW h cm-2 (41.2 Wh kg- 1) at power density of 0.6 mW cm-2 (26.3 W kg- 1), and possesses excellent volumetric capacitance and energy density of 29.3 F cm-3 and 5.8 Wh cm-3, respectively. The outstanding performance can be attributed to the excellent hierarchical structure, low tortuosity, and high phosphorus doping with fast accessible channels at high current density. The extraordinary performance of CW-P has been elucidated by density functional theory calculations, which confirm the enhanced activity by P-doping. These results demonstrate that the CW-P electrode is promising for practical application in energy storage devices and encourage more investigations for thick electrode.