Potassium metal is regarded as a promising anode material for potassium-ion batteries due to its high theoretical capacity and low redox potential. However, its performance is hindered by rapid capacity fading, primarily caused by an unstable solid electrolyte interphase (SEI) and continuous dendrite growth. Herein, by coating liquid metal (LM) alloy (GaInSn) onto copper foil, we prepared a special LM@Cu substrate, significantly improving the deposition/stripping behavior of potassium metal and thus achieving long-cycling K metal battery. The excellent potassiophilicity and electrolyte wettability of LM@Cu effectively reduce the K nucleation overpotential, promote charge transfer kinetics, and enable self-diffusive planar growth mode. Moreover, ex situ scanning electron microscopy and in situ optical microscopy analyses show that the LM coating induces uniform potassium deposition, reduces volume expansion, and achieves a dendrite-free K anode. Additionally, when 3,4,9,10-perylene-tetracarboxylic diimide (PTCDI) is employed as the cathode and K-LM@Cu (LMK) as the anode, the potassium metal battery demonstrates an initial reversible capacity of 124.4 mAh·g⁻¹. Even after 4900 cycles at a current density of 500 mA·g⁻¹, it maintains a high reversible capacity of 78.2 mAh·g⁻¹. The self-diffusive planar growth mechanism enabled by liquid metal offers a promising approach for developing practical and durable potassium metal batteries.