Photocatalytic conversion of CO_2 into renewable hydrocarbons using solar energy is one of the potential solutions to both global warming and fuel crisis. Of particular interest in this work, we prepared AgI/Ag_3PO_4 heterojunction catalyst as photocatalyst to convert CO_2 into value-added products. The synthesis route of AgI/Ag_3PO_4 involved first ion-exchange preparation of tetrahedron Ag_3PO_4 followed by precipitation of AgI. The samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), specific surface area test (BET), and ultraviolet-visible absorption spectra (UV-Vis). With the mole ratio of AgI/Ag_3PO_4 being 30%, the catalyst showed the best catalytic performance for reduction of CO_2, and the major product was CH_4. The pronounced photocatalytic performance should be attributed to the efficient separation of the light electrons and holes arisen from the p-n junction between Ag_3PO_4 and AgI.