Flower-like Pt/BiOI composite nanosheets were successfully constructed by a photo-reduction method. The nanosized Pt/BiOI composites exhibited considerably enhanced photocatalytic performance for rhodamine B (RhB) and phenol degradation under visible-light irradiation (λ > 420nm) when compared with that of pure BiOI. Various characterization methods were used to evaluate the mechanism of enhanced photocatalytic activity. The highest photocatalytic activity was achieved when the composites had a Pt content of 3wt%, which is 17.8 times and 9 times higher than that of pure BiOI, respectively. The enhanced catalytic efficiency was attributed to the low rate of recombination of electron-hole pairs. Superoxide radicals and holes were found to be crucial species during the photocatalytic degradation. A possible mechanism of photocatalysis was also proposed based on the results of experiments.