We employ optical pump-terahertz probe spectroscopy to investigate the composition-dependent photoconductivity in ternary CdS_xSe_1–x nanobelts. The observed carrier dynamics of CdS nanobelts display much shorter lifetime than those of ternary CdS_xSe_1–x nanobelts. This indicates the implementation of CdS nanobelts as ultrafast switching devices with a switching speed potentially up to 46.7 GHz. Surprisingly, ternary CdS_xSe_1–x nanobelts are found to exhibit much higher photoconductivity than binary CdS and CdSe. This is attributed to the higher photocarrier densities in ternary compounds. In addition, the presence of Se in samples resulted in prominent CdSe-like transverse optical (TO) phonon modes due to electron–phonon interactions. The strength of this mode shows a large drop upon photoexcitation but recovers gradually with time. These results demonstrated that growth of ternary nanostructures can be utilized to alleviate the high surface defect density in nanostructures and improve their photoconductivity.