The invention provides a composite photocatalytic water splitting catalyst and a preparation method thereof. Pt-based alloy is composited on a prepared ZnxCd<1-x>S semiconductor material to obtain the composite photocatalytic water splitting hydrogen production catalyst. By adopting the preparation method, the semiconductor can be tightly combined with metal, so that the catalytic hydrogen production efficiency of the semiconductor can be improved; the prepared photocatalytic water splitting hydrogen production catalyst has good photocataytic hydrogen production effect under the radiation of the light, and the photocatalytic stability is high. In addition, Pt-based alloy is used for substituting pure Pt as assistant catalyst, so that the hydrogen production efficiency of the catalyst is improved, the consumption of precious metal is also reduced, and important significance on developing and utilizing the solar hydrogenation production technology can be realized.