Using high-capacity alloy anodes can grandly advance potassium-ion batterie. However, one common issue plaguing these anodes is the loss of active K⁺ ions associated with persistent side reactions on unstable electrode-electrolyte interfaces, resulting in low initial Coulombic efficiency (ICE) and shortened cycling life. Herein, we first explore the potential of a solution-based potassiation reagent (potassium-biphenyl in dimethoxyethane, K-Bp/DME) for chemical prepotassiation. With a low redox potential of 0.45 V vs. K⁺/K, the K-Bp/DME solution can readily pre-storing K⁺ ions into various alloy-type anodes by a simple immersion operation in a few minutes. Taking nanostructured Bi/P/CNT composite as an example, we demonstrate that the K-Bp/DME prepotassiation simultaneously enables the notable ICE improvement (from 51.9 to 96.2%) and long-life cycling (running 700 cycles at 1.5 A g⁻¹ with a tiny capacity attenuation rate of 0.031% per cycle) by forming a KF-rich SEI film. Furthermore, the full cell assembled by coupling the prepotassiated Bi/P/CNT anode with a potassium Prussian blue cathode delivers a desirable ICE of 88.6% and superior capacity retention of 75.3% after 450 cycles at 0.3 A g⁻¹. Besides, this prepotassiation method is also successfully extended to other anodes (Sb and Sn), indicating its general applicability.