The direct Z-scheme configuration is particularly beneficial for improving the activity of a photocatalyst by fully exploring the superior oxidation or reduction capability of both coupled components of a heterojunction. Herein, a composite of BiOI quantum dots (QDs) and g-C3N4 was fabricated through a precursor in situ transformation method at ambient temperature using a Bi2O3/g-C3N4 composite as the precursor. By reducing the size of the BiOI to the quantum scale, full and intimate contact between the BiOI and g-C3N4 was achieved. In the photocatalytic activity test under visible light, the as-prepared BiOI/g-C3N4 composite outperformed the individual BiOI and g-C3N4. The Z-scheme electron migration played a key role in the performance enhancement of the composite, because carriers with stronger redox capacity were maintained, while the holes and electrons with weaker redox capacity recombined. This research may also help to elucidate the design of photocatalysts.