Monolayer molybdenum disulfide (MoS₂) has weak light absorption due to its atomically-thin thickness, thus hindering the development of MoS₂-based optoelectronic devices. CdS_xSe${}_{1-x}$ 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 CdS${}_{0.42}$Se${}_{0.58}$ 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 /CdS${}_{0.42}$Se${}_{0.58}$ nanobelt photodetector has a wide spectral response range (400–800$$nm), high responsivity (527.22$$A/W) and large external quantum efficiency (EQE) ($1.06\times 10^5$%). Compared with the isolated monolayer MoS₂ nanosheet, both the responsivity and EQE of the hybrid photodetector are increased by 117.4 times under 620$$nm illumination. This study provides a way to prepare hybrid photodetectors with wide spectral response and high responsivity.