In this study, Ag₃PO₄@holmium phosphate (HP) composite photocatalysts were first prepared by silver-ammonia-solution-assisted solution co-deposition. During co-deposition, Ag₃PO₄ particles were encapsulated by a flocculent amorphous substance layer of HP, affording a Ag₃PO₄@HP core–shell heterojunction structure with a Ho/Ag molar ratio of 0.05/2.95. The core–shell structure significantly improved the photocatalytic activity of Ag₃PO₄. In addition, the photocatalytic mechanism of Ag₃PO₄@HP was discussed.