Controllable synthesis of hollow structure still remains a huge challenge. g-C3N4 hollow nanocubes with (002) facets exposed (c-CN) are for the first time prepared by calcining cyanuric acid-melamine in the presence of KCl. It is proposed that the c-CN growth is templated by cubic-phase KCl cage at molten state, then Oswald ripening leads to the hollowing of nanocubes. Moreover, silver is loaded on c-CN and its electrochemistry properties are investigated. It is found that at 0.05 mA/cm2, Ag/c-CN electrode shows a higher specific capacitance (10.44 mF/cm2) than that (3.79 mF/cm2) of Ag/p-CN (Ag loading on particle-like g-C3N4). The higher capacitance of Ag/c-CN is mainly attributed to the unique hollow nanocube morphology. On the one hand, c-CN has a larger BET surface area (19.8 m2/g) and a smaller electrical resistance (121.4 Ω), compared to p-CN (4.4 m2/g, 530.8 Ω). On the other hand, theory calculation indicates that the electrolyte cation (e.g., Na+, K+) is easier to desorb from (002) facet than from (100) facet. Hence, the exposed (002) facet of c-CN benefits the cation desorption, improving charging-discharging kinetic processes. Furthermore, after 2500 cycles, ~100% capacitance of Ag/c-CN is retained. It is proposed that the internal and external surfaces of hollow nanocubes can provide more accommodation sites for Ag, favoring the uniform distribution of Ag. Therefore, Ag/c-CN exhibits excellent electrochemical capacitance and cycle stability. Additionally, Ag/[email protected]//[email protected] asymmetric supercapacitor (ASC) displays a high energy density of 25 μWh/cm3, a high power density of 17 mW/cm3, and a long cycling lifetime (86.1% capacitance retention after 2500 charge/discharge cycles). The ASC is flexible and can be embedded into other power supply devices.