Building step-scheme (S-scheme) heterojunctions has recently emerged as a highly effective approach for developing superior photocatalysts for water purification. Herein, a C3N5/ Ag3PO4 (CA) S-scheme heterojunction was prepared by in situ growth of Ag3PO4 nanoparticles on 2D C3N5 nanosheets. Notably, under visible-light irridiation, CA exhibited significantly higher activity in the photodegradation of LEVO, which is about 28.38, 2.41, and 2.14 times higher than the rates for C3N5, Ag3PO4, and the mixture, respectively. Based on the radical scavenging experiments, the mechanism for enhanced photocatalytic performance has been analyzed, is attributed to improved interfacial charge separation, the elevated redox potential of photon-generated electrons and holes, and the increased generation of active species resulting from the S-scheme transfer of photoinduced carriers. Additionally, CA demonstrates greater stability than either C3N5 or Ag3PO4 alone in the photo-oxidation of LEVO and the photodegradation of RhB. In essence, this study not only deepens our comprehension of the photocatalytic mechanism of CA, but also pioneers a novel concept for the development of highly effective and stable S-type heterojunction photocatalysts.