As anodes for metal-ion batteries, metal phosphides usually suffer from severe capacity degradation because of their huge volume expansion and unstable solid electrolyte interphase (SEI), especially for potassium-ion batteries (PIBs). To address these issues, this study proposes amorphous phosphates acting as buffer materials. Ten types of metal phosphide composites embedded with in situ-formed amorphous phosphates are prepared by one-step ball milling using red phosphorus (RP) and the corresponding metal oxides (MOs) as starting materials. A zinc phosphide composite is selected for further study as a PIB anode. Benefitting from the effective suppression of volume variation, a KF-rich SEI is formed on the electrode surface in the KFSI-based electrolyte. The zinc phosphide composite exhibits a high reversible specific capacity and outstanding long-term cycling stability (476 mAh g over 350 cycles at 0.1 A g after going through a rate capability test and 177 mAh g after 6000 cycles at 1.0 A g) and thus achieves the best cycling performance among all reported metal phosphide-based anodes for PIBs. This study highlights a low-cost and effective strategy to guide the development of metal phosphides as high-performance anodes for PIBs.