With the demand for carbon dioxide emission reduction, the sustainable conversion of useless biomass into high-value energy storage devices has received excellent scientific and technological attention. The high synthesis cost and low specific capacitance limited the supercapacitor application. Therefore, biomass-derived sulfur-doping porous carbon (SPC) has been synthesized from ulothrix using simple pyrolysis and chemical activation methods. The unique activated carbon material exhibits a high specific surface area (2490 m2 g-1), and the effect of the activator addition ratio was systematically investigated. The optimized SPC-2 displayed a high specific capacitance (324 F g-1 at 1 A g-1) and excellent cycling stability (90.6% retention after 50 000 cycles). Furthermore, a symmetric supercapacitor (SSC) based on SPC-2 demonstrated a high energy density (12.9 Wh kg-1) at an 800 W kg-1 power density. This work offers a simple, economical, and ecofriendly synthetic strategy of converting widespread, useless biomass waste into high-performance supercapacitor applications.