Myriad materials with desirable functional property resulting from their unique structures ignite enormous interest in synthesizing materials with controlled structural morphology toward achieving novel or enhanced properties for target applications. Herein, the novel and unique two-dimensional (2D) MoS2 nanosheet-coated Bi2S3 discoids composites, which feature a Bi2S3-core/MoS2-shell structure, have been elaborated via a facile anion-exchange strategy. Using the MoS2 nanosheets to coat the surface of Bi2S3 discoids boosts the light-harvesting efficiency and charge separation and promotes faster charge transport and collection, thus leading to the higher activity of the photocatalytic reduction of Cr(VI) under visible light irradiation (lambda > 400 nm). In particular, the phase evolution and possible formation mechanism of the MoS2-Bi-2-S-3 coreshell structure have been explored by virtue of temperature- and time-dependent experiments. It is anticipated that this work could promote further interest in adopting an anion-exchange strategy to fabricate semiconductor-based composite materials with controlled architectural morphology and enhanced photocatalytic performance toward diverse applications.