The 'dry' activation by mixing precursor and activation agent, is considered as a traditional method to prepare porous carbon. Herein, we develop a distinct 'wet' activation approach, that is, synergistic supercritical water and KOH activation to prepare porous carbon from pistachio shell. The activation process is less activation agent-consuming and time-costing, highly efficient and free of harmful products, in accord with the principles of green chemistry. The sample presents a microporous structure and medium specific surface area (200-900 m(2) g(-1)) with a partial graphitization confirmed by X-ray Difffraction and Raman. The sample also exhibits stable surface groups. The samples are evaluated as the potential electrode material for supercapacitor. The sample S1 shows excellent electrochemical performance, delivering a high capacitance of 232 F g(-1) (260 F cm(-3)) at 0.5 A g(-1), and an impressive 144 F g(-1) (161 F cm(-3)) at 100 A g(-1), as well as long cyclic capability (>10000 cycles). Remarkably, it also possesses a surface area normalized capacitance as high as 65.5 mu F cm(-2). The two-electrode symmetric cell also demonstrates its potential practical performance. This method provides a promising route to achieve the green, cost-effective, and large-scale preparation of porous carbon from biomass for energy storage. (C) 2018 The Electrochemical Society.