2D alloy‐based anodes show promise in potassium‐ion batteries (PIBs). Nevertheless, their low tap density and huge volume expansion cause insufficient volumetric capacity and cycling stability. Herein, a 3D highly dense encapsulated architecture of 2D‐Bi nanosheets (HD‐Bi@G) with conducive elastic networks and 3D compact encapsulation structure of 2D nano‐sheets are developed. As expected, HD‐Bi@G anode exhibits a considerable volumetric capacity of 1032.2 mAh cm−3, stable long‐life span with 75% retention after 2000 cycles, superior rate capability of 271.0 mAh g−1 at 104 C, and high areal capacity of 7.94 mAh cm−2 (loading: 24.2 mg cm−2) in PIBs. The superior volumetric and areal performance mechanisms are revealed through systematic kinetic investigations, ex situ characterization techniques, and theorical calculation. The 3D high‐conductivity elastic network with dense encapsulated 2D‐Bi architecture effectively relieves the volume expansion and pulverization of Bi nanosheets, maintains internal 2D structure with fast kinetics, and overcome sluggish ionic/electronic diffusion obstacle of ultra‐thick, dense electrodes. The uniquely encapsulated 2D‐nanosheet structure greatly reduces K+ diffusion energy barrier and accelerates K+ diffusion kinetics. These findings validate a feasible approach to fabricate 3D dense encapsulated architectures of 2D‐alloy nanosheets with conductive elastic networks, enabling the design of ultra‐thick, dense electrodes for high‐volumetric‐energy‐density energy storage. A 3D dense encapsulated architecture of 2D Bi nanosheets is designed. It displayed superior 3D structural stability, large volumetric capacity (1032.2 mAh g−1), ultra‐high loading (24.2 mg cm−2), large areal capacity (7.94 mAh cm−2), and long lifespan of 2000 cycles (75.0% retention) in potassium‐ion batteries.