Phosphorus-based anodes for alkali metal-ion batteries are attractive due to their high theoretical-specific capacity. However, their poor electrochemical performance caused by relatively large volume variations during cycling, low electrical conductivity, and severe electrolyte decomposition due to highly reactive phosphide surface hinder their potential applications. Herein, we confine Sn 4 P 3 in N-doped carbon fibers as anode for potassium-ion batteries with enhanced cycling stability and high rate capability (160.7 mA hr g −1 after 1,000 cycles at 500 mA g −1 ). The Sn 4 P 3 anodes undergo a sequential conversion (P to K 3 P 11 , K 3 P) and alloying (Sn to KSn) reactions with synergistic K-storage mechanisms. Also, the electrolyte with potassium bis(fluorosulfonyl)imide salt can effectively suppress the dendrite growth in K stripping/plating, stabilize the solid-electrolyte interphase (SEI) layer, and avoid excessive side reactions, thus enhancing the electrode stability. This work provides a feasible approach to overcome the durability bottlenecks of K-ion batteries through regulating dendrite growth and SEI formation. Potassium, with abundant resources and a low standard hydrogen potential close to that of lithium, makes the potassium-ion battery an alternative candidate to replace the lithium-ion battery in large-scale energy storage applications. Nevertheless, critical problems related to the large volume changes during electrochemical cycling remain a challenge due to the large size of the potassium ions. Among the anode candidates, phosphorus-based anodes for alkali-metal-ion batteries are attractive due to their competitively high energy density. Herein, we used carbon fibers to confine ball-milled Sn 4 P 3 particles to buffer the volume changes, thus improving the cycling stability of Sn 4 P 3 anode. This design offers a feasible avenue for scalable fabrication of phosphorus-based electrode materials achieving a high-energy density without sacrificing its cycling stability for alkali-metal-ion batteries. This work provides a feasible approach to overcome the durability bottlenecks of potassium-ion batteries (PIBs) through regulating dendrite growth and solid-electrolyte interphase formation. In operando synchrotron XRD studies suggested that Sn 4 P 3 anodes undergo a sequential conversion (P to K 3 P 11 , K 3 P) and alloying (Sn to KSn) reactions with a synergistic K-storage mechanism. We hope that this work inspires robust developments in exploring phosphorus-based anode materials in PIBs through materials design and manipulation of the salt/additive chemistry of the electrolytes.