Improving separation efficiency of carriers and inhibiting the inverse reaction are key challenges for achieving efficient sunlight driven conversion of CO2 by H2O as reducing agent into CH4. Here, we proposed using metal Bi with low work function to modify n-type Ta3N5, thus building an ohmic junction photocatalyst of Ta3N5/Bi. It achieved an about 5 times increase in CH4 yield compared with Ta3N5. The enhanced photocatalytic activity was ascribed to the following two effects: (1) The junction electric field drove the injection of conduction-band electrons of Ta3N5 to Bi, greatly improving separation efficiency of carriers; (2) The oxidation and reduction reaction sites were respectively constructed over Ta3N5 and Bi, effectively separating the oxidation reaction of H2O and the reduction reaction of CO2, which distinctly suppressed the reverse reaction during CH4 generation.