Interconnected porous carbons show great potential for high-rate capacitive energy storage, especially at high electrode mass loadings, due to their continuous conductive network and ion migration channels. Herein, we show a simple construction of 3D interconnected porous carbon through spontaneous merging of nano-sized ZIF-8 polyhedrons (ca. 76 nm) during the pyrolysis process. The obtained porous carbon shows a continuous conductive network, interconnected micro-/meso-porous structure, abundant N and O heteroatoms, and good hydrophilicity, enabling fast electron and ion transport kinetics for thick electrodes. As a result of the above merits, the as-obtained carbon shows high capacitance (320.7 F g⁻¹ at 1 A g⁻¹) and outstanding rate performance (213.9 F g⁻¹ at 100 A g⁻¹). Importantly, even at a high mass loading of 15 mg cm⁻², the as-prepared material exhibits an outstanding rate performance of 137.7 F g⁻¹ at 100 A g⁻¹. Furthermore, the assembled symmetric supercapacitor using 1 mol L⁻¹ Na₂SO₄ electrolyte shows a high energy density of 20.6 W h kg⁻¹ at 0.5 kW kg⁻¹ and 7.6 W h kg⁻¹ at 34.3 kW kg⁻¹, as well as good cycle stability. Our work indicates that building 3D interconnected structure by merging nanoparticles shows great potential for high-mass-loading energy storage.