 Hydrothermal carbonization (HTC) process of biomass has showed many merits for the preparation of unique carbon-based material. To overcome the challenges of inherently low porosity and conductivity associated with HTC method-prepared carbons, herein a modified hydrothermal reaction was employed to prepare hierarchically porous carbon spheres (HPCS) with large surface area by using starch as carbon precursor and ammonium ferrous sulfate as porogen. The HPCS materials exhibit excellent electrochemical performance as active materials for electric double-layer capacitors, as demonstrated by a large specific capacitance of 248 F g−1 at 0.5 A g−1, a high volumetric capacitance of 181 F cm−3 at 0.5 A g−1, good capacity retention, and excellent cycling stability. The modified hydrothermal method proposed here is promising to prepare carbon materials for wide applications.