Birnessite M_xMnO₂ (M = Na⁺, K⁺, etc.) has emerged as a promising alternative to the classical MnO₂ material owing to its improved pseudocapacitive performance for energy storage. Understanding their structure–property correlation is essential for the development and application of advanced supercapacitors. Herein, we adopt the crystal field theory and density functional simulation to reveal the structural dependence of the pseudocapacitive property of K_xMnO₂. Attributing to the Jahn–Teller effect of Mn³⁺, the bandgap of K_xMnO₂ can be tuned by changing the x value (i.e., the Mn(Ⅲ)/Mn(Ⅳ) ratio). Then, we design a narrow-bandgap K_0.25MnO₂ (0.84 eV), which affords a high capacitance of 415 F g⁻¹ at 1 A g⁻¹ and a desirable rate capability of 293 F g⁻¹ at 20 A g⁻¹. Operando Raman spectroscopy confirms that the Jahn–Teller induced structure evolution of [MnO₆] octahedron accounts for the superior pseudocapacitive behavior of K_0.25MnO₂. This finding offers theoretical guidance to the design and application of birnessite materials for pseudocapacitors.