Nanostructured birnessite with tunneled structures and nearly ideal capacitive behaviors are attractive as electrode material for aqueous asymmetric supercapacitors (ASCs). However, their practical application is hindered by inadequate structural stability, sluggish reaction kinetics, and the lack of deeper understanding of electrochemical mechanisms. Herein, oxygen defect modulated sodium‐intercalated manganese oxides (Na_0.55Mn₂O_4‐x·1.5H₂O, abbreviated as NMO_x) with intercalating sodium ions and water crystals are massively fabricated, which can enable fast diffusion of cations and good structural stability. Systematical in situ and ex situ characterizations verify that the preeminent capacitive charge storage of NMO_x is governed by interlayer cation intercalation and deintercalation, accompanied by interlayer spacing expansion/contraction. Subsequently, a horizontally oriented carbon nanotube microfilm with outstanding electrical conductivity and electrolyte wettability is reported for aqueous ASCs, which exhibits a wide operating voltage (2.4 V), a high energy density of 88.9 W h kg⁻¹, and a superb cycle performance (92.7% retention after 50 000 cycles). Furthermore, a flexible planar ASC is prepared with landmark volumetric energy/power densities (60.2 mW h cm⁻³, 23.7 W cm⁻³), and excellent mechanical flexibility. This study provides not only an effective approach to fabricate tailoring structure and remarkable electrochemical properties of birnessite material, but also a deeper understanding of the charge storage mechanisms.