The absorption-emission spectra of CdSxSe1-x/ZnS quantum dots (QD) dispersed in the aqueous solution and their photoluminescence (PL) intensities varying with time are measured using an ultraviolet-visible-near infrared spectrophotometer and steady state/transient PL spectrometer, respectively. The variation in the PL lifetime with the QD size, x, and temperature is obtained. The measured PL lifetime is mainly decided by the interband-direct transition of QD, and the indirect transition of defect states is the second factor. Empirical formulas are presented for PL-peaking wavelength and PL lifetime depending on the QD size and x. The results demonstrate that the PL lifetime increases as the increasing particle size and decreases as the increasing proportion of the S component. However, it is insensitive to temperature. The measured PL lifetime is approximately 2.51-3.22 μs for the QDs with the size of 4.06-9.22 nm when the x is 9.45-0.366 and the temperature is 15-55 ℃.