Photocatalytic water-splitting employing the Z-scheme semiconductor systems mimicking natural photosynthesis is regarded as a promising way to achieve efficient soalr-to-H2 conversion. Nevertheless, it still remains a big challenge to design high-performance direct Z-scheme photocatalysts without the use of noble metals as electron mediators. Herein, a unique Cd0.5Zn0.5S/WO3-x direct Z-scheme heterojunction was constructed for the first time, which consisted of smaller O-vacancy-decorated WO3-x nanocrystals anchoring on Cd0.5Zn0.5S nanocrystals with S vacancies and zinc blende/wurtzite (ZB/WZ) twinning superlattices. Under visible-light (λ > 420 nm) irradiation, the Cd0.5Zn0.5S/WO3-x composites exhibited an outstanding H2 evolution reaction (HER) activity of 20.50 mmol h-1 g-1 (corresponding to the apparent quantum efficiency of 18.0% at 420 nm), which is much superior to that of WO3-x, Cd0.5Zn0.5S, and Cd0.5Zn0.5S loaded with Pt. Interestingly, the introduced O and S vacancies contributed to improving the HER activity of Cd0.5Zn0.5S/WO3-x significantly. Moreover, the cycling and long-term HER measurements confirmed the robust photocatalytic stability of Cd0.5Zn0.5S/WO3-x for H2 production. The excellent light harvesting and efficient spatial charge separation induced by the ZB/WZ twinning homojunctions and defect-promoted direct Z-scheme charge-transfer pathway are responsible for the exceptional HER capability. Our study could enlighten the rational engineering and optimization of semiconductor nanostructures for energy and environmental applications.