The conversion reaction can provide a high specific capacity through a multiple electron transmission in redox reaction, but there are few studies on the conversion-reaction-based cathode materials in sodium ion batteries (SIBs) currently. The Cu3(PO4)2 material based on conversion reaction has gradually attracted researchers' attention due to its high specific capacity, good thermal stability, etc. However, the reversibility of Cu3(PO4)2 in the conversion reaction process is poor, resulting in the short cycle life. To solve this problem, the Cu3(PO4)2 with different thickness of amorphous layer was synthesized by controlling the solid phase reaction temperature. It was found that the Cu3(PO4)2 with a 3 nm amorphous layer showed an enhanced cyclic stability. The reversible capacity remained at 112 mAh g−1 after 30 cycles under a current density of 0.1 A g−1. Further electrochemical characterization showed that the suitable thickness of amorphous layer can maintain the reaction products Na3PO4 and Cu a good electrical contact and bring a good charge transfer ability even after conversion reaction. It promoted the reversibility in conversion process and led to an enhanced cycle performance. The mechanism discussed in this paper may provide reference for improving the performance of other conversion-reaction-based materials.