The photoluminescence (PL) lifetimes of PbS quantum dots (QDs) with different sizes, temperatures, and backgrounds are determined by the measurements of absorption spectra, time-resolved PL spectra, and transmission electron microscopy (TEM) images of PbS QDs. An empirical formula describing the variation of the first absorption-peak wavelength with QD size is obtained. The results indicate that the PL lifetime is strongly correlated to QD size in a way of negative exponential-dependence; the lifetime is weakly dependent on temperature; background material can affect the lifetime due to the surface-polarization effect.