Monolayer molybdenum disulfide (MoS₂) has weak light absorption due to its atomically-thin thickness, thus hindering the development of MoS₂-based optoelectronic devices. CdS_xSe1−x has excellent photoelectric performance in the visible light range, and its nanostructure shows great potential in new nanoscale electronic and optoelectronic devices. In this work, a composite photodetector device with the combination of monolayer MoS₂ nanosheets and CdS0.42Se0.58 nanobelts has been successfully prepared, which can not only maintain the inherent excellent properties of the two blocks, but also play a synergistic role between them, thus improving the photoelectric performance of the device. The monolayer MoS₂ nanosheet /CdS0.42Se0.58 nanobelt photodetector has a wide spectral response range (400–800nm), high responsivity (527.22A/W) and large external quantum efficiency (EQE) (1.06×105%). Compared with the isolated monolayer MoS₂ nanosheet, both the responsivity and EQE of the hybrid photodetector are increased by 117.4 times under 620nm illumination. This study provides a way to prepare hybrid photodetectors with wide spectral response and high responsivity.