Two-dimensional MoS2 has been widely used as hydrogen evolution reaction (HER) cocatalyst to load onto nanostructured semiconductors for visible light-response photocatalytic hydrogen production. However, its another important role as light harvester because of the band-gap tunable property and beneficial band position has been rarely exploited. Herein, few layer-thick MoS2 nanoflakes with extended light absorption over the range of 400 to 680 nm and a photocatalytic HER rate of 0.98 mmol/h/g have been obtained. Then 7-nm-sized Cd0.5Zn0.5S quantum dots (QDs) are selectively grown upon ultrathin MoS2 nanoflakes for enhanced photocatalytic H-2 generation. Upon the photocatalytic, light absorption, and charge transfer properties of the MoS2-Cd0.5Zn0.5S composites evolved with the amount of MoS2 from 0 to 3 wt%, the multiple roles of MoS2 as long-wavelength light absorber, in-plane carrier mediator, and edge site-active HER catalyst have been revealed. An optimum H-2 generation rate of 8863 mu mol/h/g and a solar to hydrogen (STH) efficiency of 2.15% have been achieved for 2 wt% MoS2-Cd0.5Zn0.5S flakes. Such a strategy can be applied to other cocatalysts with both the light response and HER activity for efficient photocatalytic property.