Noble metal co-catalysts are efficient for photocatalytic hydrogen generation from formaldehyde, but their high cost and low abundance limit their scale-up applications. Alloying a second metal with noble metals offers an available way to decrease their usage and consumption and enhances their catalytic activity. Among the potential candidates, Sn is a high abundance element in the earth. In this work, a new highly active low-cost Pd–Sn/RGO catalyst has been developed. Under optimized conditions, the hydrogen evolution rate reached up to 530.07 μmol h⁻¹ driven by visible light. By taking advantage of the optimized adsorption nature of the active sites on the alloy cocatalyst, effective separation and transfer of photogenerated charges have been achieved; at the same time, the photo-generated electrons have a longer lifetime. More interestingly, it has been found that trace amounts of oxygen are helpful in catalytic hydrogen production from HCHO.