A novel and highly efficient photocatalyst of Cd0.5Zn0.5S@ZIF-8 nanocomposite has been developed by a facile self-assembly strategy. This is the first report on the application of CdxZn1-xS and metal-organic framework (MOF) nanocomposite as photocatalysts for the reduction of Cr(VI). The resulting Cd0.5Zn0.5S@ZIF-8 exhibited higher photocatalytic activity than that of pristine Cd0.5Zn0.5S and ZIF-8. Particularly, the CZS@Z60 composite with 60 wt% of ZIF-8 exhibited a photocatalytic activity that is about 1.6 times as high as that of Cd0.5Zn0.5S. The dominant reason for the improved photocatalytic reduction potential is proved to be the newly-formed interfacial SZn bonds that firmly connect Cd0.5Zn0.5S and ZIF-8 and substantially improve the separation efficiency of photo-excited electrons and holes. The newly-formed chemical bonds are confirmed by XPS analyses, and the prolonged lifetime of photo-excited electrons is evidenced by the electrochemical measurement of photocurrent, which shows that the photocurrent on Cd0.5Zn0.5S@ZIF-8 is much higher than that of Cd0.5Zn0.5S and ZIF-8. This study clearly demonstrates that the MOF-based composite nanomaterials hold great promises for applications in the field of environmental remediation and for design of novel photocatalytic materials.