The photoluminescence (PL) intensity of free excitons in a GaInAsSb/GaAlAsSb quantum well (QW) laser structure is investigated as a function of excitation intensity and lattice temperature.The relationship between PL intensity of free excitons and excitation intensity as well as lattice temperature in the steady-state PL measurements was developed for the quantitative descriptions of the experimental data.Calculations show that the variation of exciton relative population with excitation intensity has strong influence on the relationship between PL intensity of free excitons and excitation intensity. It is found that the linear coefficient I/I0 is actually the characterization of the exciton photoluminescence efficiency. According to our model,the background density of the quantum well material as well as the scattering time constant of the QW,in addition to the excitonic binding energy and activation energy, is also obtained by fitting the Arrhenius plot of PL intensity.