The microstructure design of electrode material is a crucial point to optimize the supercapacitor’s electrochemical properties. In this work, a Bi-based nanocomposite with a three-layer structure (Bi–Bi₂O₃@carbon armour (CA)/carbon dots (CDs)) is synthesized and investigated. This material inherits high capacitance and high activity from bismuth-based materials, and the coated CA protects the structure from complete oxidization and improves surface hydrophilicity. Furthermore, CDs in CA can enhance the ion conduction efficiency between the catalyst, carbon membrane, and electrolyte. As a consequence, the specific capacitance of the electrode reaches 973 F·g⁻¹ under 1 A·g⁻¹, and the energy density achieves 32.5 Wh·kg⁻¹ with a power density of 266.9 W·kg⁻¹, with impressive electrochemical stability that Coulomb efficiency of the electrode remains about 100% after 5000 cycles. Furthermore, in-situ Fourier transformation infrared (FT-IR) analyzes the structure evolvement of the material during synthesis and finds that the annealing process of the material removes a great number of oxygen-containing groups of CA and CDs, generating oxygen vacancies, defects, and thus active sites, which enhance the capacitance of the material.