Ternary alloy CdS_xSe_1–x has the physical properties of CdS and CdSe, and its band gap can be adjusted by changing the component ratio of the elements. The alloy has excellent photoelectric properties and has potential application in optoelectronic devices. Although one has made some research progress of the CdSSe-based photodetectors, their performances are still far from the commercial requirements, so how to improve the performance of the device is the focus of current research. In this work, a single crystal CdS_0.42Se_0.58 nanobelt device is first prepared by thermal evaporation. Under 550 nm illumination and 1 V bias, the ratio of photocurrent to dark current of the device is 1.24×10³, the responsivity arrives at 60.1 A/W, and the external quantum efficiency reaches 1.36×10⁴%, and the detectivity is 2.16×10¹¹ Jones. Its rise time and fall time are about 41.1/41.5 ms, respectively. Secondly, after the CdSSe nanobelt is decorated by Au nanoislands, the optoelectronic performance of the device is significantly improved. Under 550 nm illumination and 1 V bias, the I_p/I_d ratio, responsivity, external quantum efficiency and detectivity of the device are increased by 5.4, 11.8, 11.8 and 10.6 times, respectively, and the rise time and fall time are both reduced to half of counterparts of single CdSSe nanobelt. Finally, the microscopic physical mechanism of the enhanced optoelectronic performance of the device is explained based on localized surface plasmon resonance of Au nanoislands. After the combination of gold nanoislands and CdSSe nanobelt, the difference in Fermi level between them results in the transfer of electrons from CdSSe nanobelt to Au nanoislands, thus forming an internal electric field at the interface, which is directed from CdSSe nanobelt to Au nanoislands. Under illumination, the electrons in the Au nanoislands acquire enough energy to jump over the Schottky barrier because of localized surface plasmon resonance. These photoexcited hot electrons are trapped and stored in extra energy levels above the conduction band minimum, and then are cooled down to the band edge, thus realizing the transfer of electrons from Au nanoislands to CdSSe nanobelt. Moreover, the internal electric field also greatly promotes the transfer of hot electrons from Au nanoislands to CdSSe nanobelt, and inhibits the recombination of carriers at the interface, resulting in large photocurrent. Our work provides an effective strategy for fabricating high-performance photodetectors without increasing the device area.