Layered heterostructures have gained significant attention as potential platforms for energy storage because of their unique electronic and interfacial characteristics. Engineering heterojunctions in a rational, efficient, and effective manner is critical to well-defined battery systems. Herein, FePSe₃–FeSe₂ heterojunctions uniformly dispersed in a Ketjen black carbon matrix (FePSe₃–FeSe₂/C) were prepared by a one-step phosphorization–selenization method. The conductive carbon matrix can prevent the aggregation of FePSe₃–FeSe₂ hexagonal nanoplatelets. Moreover, theoretical calculations demonstrate that FePSe₃–FeSe₂ heterostructures with a strong asymmetric electric field can facilitate K⁺ intercalation. Notably, FePSe₃–FeSe₂/C with rich active sites would undergo negative fading, which results in a capacity increase upon cycling. As an anode for potassium ion batteries (PIBs), FePSe₃–FeSe₂/C can deliver high capacities of 352 mA h g⁻¹ at 0.1 A g⁻¹ after 100 cycles and 224 mA h g⁻¹ at 1.0 A g⁻¹ after 3700 cycles. The strategy of fabricating FePSe₃–FeSe₂ heterojunctions can be extended to other metal phosphorus trichalcogenides to improve their performance in energy storage. Furthermore, the obtained results provide important insights into hetero-nanostructure design toward fast reaction kinetics for PIBs.