Here,an agricultural waste (the stem pith of helianthus annuus,SPHA) is firstly used as the precursor for preparing three-dimensional (3D) porous carbon sponge (PCS).The as-prepared 3D PCS (SPHA-700) pos-sesses unique sponge-like structure,large specific surface area (SSA) and high nitrogen doping level (4.52 at.％),which benefit the enhancement of conductivity (5.8 S cm-1) and wettability.As a binder-free elec-trode for solid-state symmetric supercapacitor,SPHA-700 delivers a relatively high specific capacitance of 137.1 F g-1 at 0.5 A g-1.Moreover,activated SPHA-700 (SPHA-ac-700-2) displays an even higher specific capacitance (403.6 F g-1 at 0.5 A g-1) in 6.0 M KOH electrolyte.The SPHA-ac-700-2-based symmetrical supercapacitor can offer high specific capacitance (271 F g-1 at 1 A g-1) and good rate capability (82.1％of capacitance retention at 1-80 A g-1) in 6.0 M KOH electrolyte,together with high energy density(23.3 Wh kg-1 at 450 W kg-1) in 1.0 M Na2SO4 electrolyte.Such excellent performance of SPHA-ac-700-2 is believed to have originated from the crushed sponge-like structure,O/N-co-doping (10.6 at.％ O and 3.3 at.％ N),high SSA,large total pore volume,and hierarchical pore structure.