Potassium-ion batteries (PIBs) are becoming one of the promising alternative metal-ion battery systems to lithium-ion batteries due to the abundance and low cost of potassium. Among the anode materials for the metal-ion battery systems, phosphorus-based materials are attractive due to the low cost and their high theoretical-specific capacity. Herein, for the first time, we report the selenium -phosphorous-carbon (Se–P–C) amorphous composites prepared by the plasma-assisted ball milling method as the high-performance anode material for PIBs. In particular, when the amorphous Se–P composite anode with the molar ratio of 1:2 and the P-milling time of 30 h (Se–2P/C@30 h), the associated PIB delivers a high reversible capacity of 634 mA h g−1 at a current density of 0.05 A g−1 and excellent rate capability of 248.6 mA h g−1 at a current density of 1 A g−1. Furthermore, the experimental characterization and analysis reveal the reaction mechanisms that K–P (K2P3) and K–Se (KSe) phases are formed during the potassiation process. The present work provides a facile approach to achieve a promising anode material for K-ion batteries, and the preparation route can be viewed as a reference for the further development of phosphorous-based anodes with high capacity and long cycle life for PIBs.