The conversion of diluted CO into tunable syngas via photocatalysis is critical for implementing CO reduction practically, although the efficiency remains low. Herein, we report the use of graphene-modified transition metal hydroxides, namely, NiCo-GR, for the conversion of diluted CO into syngas with adjustable CO/H ratios, utilizing Ru dyes as photosensitizers. The Ni(OH)-GR cocatalyst can generate 12526 μmol g h of CO and 844 μmol g h of H, while the Co(OH)-GR sample presents a generation rate of 2953 μmol g h for CO and 10027 μmol g h for H. Notably, by simply altering the addition amounts of nickel and cobalt in the transition metal composite, the CO/H ratios in syngas can be easily regulated from 18:1 to 1:4. Experimental characterization of composites and DFT calculations suggest that the differing adsorption affinities of CO and HO over Ni(OH)-GR and Co(OH)-GR play a significant role in determining the selectivity of CO and H products, ultimately affecting the CO/H ratios in syngas. Overall, these findings demonstrate the potential of graphene-modified transition metal hydroxides as efficient photocatalysts for CO reduction and syngas production.