The conversion of CO_2 into organic compounds is a promising method to mitigate global warming and assist in sustaining energy resources. A series of plasmonic photocatalysts, comprised of Ag supported on Ag_2WO_4 (Ag/Ag_2WO_4) with different crystalline phases, are fabricated by a facile ion-exchange method and subsequent reduction with hydrazine hydrate. The catalysts are characterized using X-ray diffraction (XRD) spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS), UV-Vis absorption spectroscopy, and Brunauer-Emmett-Teller analyses. Compared with Ag_2WO_4, the Ag/Ag_2WO_4exhibits a markedly improved quantum yield (QY), energy returned on energy invested (EROEI), and turnover number (TON) for CO_2 reduction to CH_4 under visible-light irradiation. Among Ag/alpha-Ag_2WO_4, Ag/beta-Ag_2WO_4, and Ag/gamma-Ag_2WO_4 catalysts, the highest activity for CO_2 photoreduction to CH_4 is obtained for Ag/beta-Ag_2WO_4 with an actual molar composition of 4% Ag and 96% Ag_2WO_4. Correspondingly the QY, EROEI, TON, and pseudo first-order rate constant are 0.145%, 0.067%, 9.61, and 1.96 * 10~(-6) min~(-1), respectively. Moreover, the plasmonic Ag/Ag_2WO_4 photocatalysts are stable after repeated reaction cycles under visible-light irradiation. It is proposed that the localized surface plasma resonance effect of surfacedeposited Ag contributed to the enhanced activities and stabilities of the Ag/Ag_2WO_4 photocatalysts.