Cluster-like Ag3PO4 nanostructures including nanoparticles, trisoctahedrons, tetrahedrons and tetrapods have been prepared by the synergetic reaction of Ag nanocrystals, phosphate anions and hydrogen peroxide. The acidity and alkalinity of the reaction solution are tuned to adjust the oxidizing ability of H2O2, and thus control the final morphology. Ag nanocrystals function as a sacrificial precursor, leading to the generation of cluster-like nanostructures. Through a kinetic study, the formation of Ag3PO4 nanocrystal clusters can be understood as the conversion from Ag to Ag3PO4 nanocrystals assisted by H2O2, followed by the oriented attachment of nanocrystals into cluster-like colloids with specific shapes. The as-prepared Ag3PO4 nanostructures have higher photocatalytic activity than commercial TiO2 and some reported Ag3PO4 microcrystals in the degradation of dyes. The catalytic activity decreases in the order nanoparticles > trisoctahedrons > tetrahedrons > tetrapods, while the stability increases in the order nanoparticles < tetrahedrons < trisoctahedrons < tetrapods, which can be explained by the extent of absorption of visible light and structural factors, including size and exposed crystal facets.