In order to enhance the fluorescence intensity and reliability of CdS nanoparticles, synthesis, fluorescence property of CdS by different mass ratio of Cd to S, and influence of stabilizer were studied. CdS/ZnO composite structures were synthesized in alkaline condition using the hydrothermal synthesis method. In addition, all samples were tested by XRD, fluorescence spectroscopy and SEM. The results showed that CdS nanoparticles and CdS/ZnO composite nanoparticles were single and relatively pure. ZnO coated on the surface of CdS. Under the 328.5 nm ultraviolet excitation, emission spectrum was narrow and symmetrical, and the emission peak was at 463 nm. The fluorescence intensity of CdS/ZnO composite structure nanoparticles increased obviously. The best mass ratio of CdS to ZnO was 1∶1, whose fluorescence efficiency was 11% higher than that of CdS nanoparticles. The results of first principle study indicated that the energy band of Cd-4d, S-3p and Cd-5s were comprised of 5, 3 and 1 energy levels, respectively, in the band structure of CdS. By comparing partial density of states in different orbits, it can be seen that the boundary of conduction band was mainly comprised of Cd-5s orbit, the boundary of valence band was mainly comprised of S-3p orbit, and the electronic states near -7 eV was mainly comprised of Cd-4d orbit. In the band structure of ZnO, valence band on top was mainly comprised of O-2p electron, the region near Fermi level was mainly comprised of Zn-3d electron, conduction band was mainly comprised of Zn-3d and O-2p electron. In CdS/ZnO composite structures, energy levels of Zn-3d electron were near energy levels of S-3p electron and presented a type-Ⅱ band structure, thus narrowing band-gap. The electron transition became easier, depressing the recombination of electrons and holes, and improving the efficiency of fluorescence.