Porous carbon has been one desirable electrode material for supercapacitors, but it is still a challenge to balance the appropriate mesopore volume and a high specific surface area (SSA). Herein, a good balance between a high SSA and mesopore volume in biomass-derived porous carbon is realized by precarbonization of wheat husk under air atmosphere via a chloride salt sealing technique and successive KOH activation. Due to the role of molten salt generating mesopores in the precarbonized product, which can further serve as the active sites for the KOH activation to form micropores in the final carbon material, the mesopore–micropore structure of the porous carbon can be tuned by changing the precarbonization temperature. The appropriate amount of mesopores can provide more expressways for ion transfer to accelerate the transport kinetics of diffusion-controlled processes in the micropores. A high SSA can supply abundant sites for charge storage. Therefore, the porous carbon with a good balance between the SSA and mesopores exhibits a specific gravimetric capacitance of 402 F g⁻¹ at 1.0 A g⁻¹ in a three-electrode system. In a two-electrode symmetrical supercapacitor, the biomass-derived porous carbon also delivers a high specific gravimetric capacitance of 346 F g⁻¹ at 1.0 A g⁻¹ and a good cycling stability, retaining 98.59% of the initial capacitance after 30,000 cycles at 5.0 A⁻¹. This work has fundamental merits for enhancing the electrochemical performance of the biomass-derived porous carbon by optimizing the SSA and pore structures.