Cu-doped CdS nanocrystals (NCs) have been prepared by wet chemical method in ODE solvent. The shape, crystal structure, and their optical properties have been studied through TEM image, X-ray diffraction pattern, absorption spectra, fluorescence spectroscopy and fluorescence lifetime. The research results show that Cu-doped CdS NCs have a spherical shape, narrow size distribution and zincblende structure when changing Cu concentration. The Cu-doped analysis showed that when Cu concentration increased, the particle size did not change. The fluorescence intensity increased as Cu concentration increasing from 0 to 10% and decreased when Cu concentration was up to 20%. The dependence of bandgap energy, emission intensity of CdS and Cu-doped CdS NCs in the temperature range of 15-300K, and the mechanism of energy transfer between the CdS substrate and Cu-doped have been investigated and explained. The time-resolved fluorescence lifetime spectrum shows that the Cu-doped CdS NCs have a very long lifetime (up to 585 ns), which are suitable for applications in the fields of biomarkers, optical amplification and photovoltaic.