The photoluminescence property of CdS/PAMAM nanocomposites in water at different ratio of Cd2+ to PAMAM(r) was studied by fluorescence emission spectrum, and the according mechanism was explored. The results indicate that in the irradiation of UV light at the wavelength of 330 nm, two peaks were shown in the fluorescence emission spectrum of CdS/PAMAM nanocomposites. One peak was attributed to PAMAM dendrimers, and the another came from CdS QDs. The strength of the frontier fluorescence emission peak became weaker and weaker, and at last disappeared with the enhancement of the ratio of Cd2+ to PAMAM; the strength of the latter fluorescence emission peak became stronger, which indicates that the electron at valence band of N atom in amines of dendrimers was transferred to CdS QDs. Oil latent finger marks deposited on transparent adhesive tapes surface treated with pure PAMAM solution with blue photoluminescence was not clear enough because of the blue light emitted from the impurity in adhesive tapes; Latent finger marks emitted strong yellow light and were successfully detected with good resolving rate with CdS/dendrimer solution with r of 10, which shows good reference value for detecting finger marks on surfaces with impurities emitting different photoluminescence colors.