A linear empirical formula of the absorption-peak wavelength depending on quantum dots (QDs) diameters is presented by measuring the near IR-absorption spectra of PbSe QDs with three diameters (4.5, 5.0, 5.6 nm). Both the absorption cross-section and absorption coefficient of QDs varying with the doping concentration and wavelength are determined by absorption-spectrum method and according to Lambert-Beer′s law. There is evidence to show that the absorption is weak dependent on the doping concentration, which can be expressed by an exponential decrease with the increased doping concentration. The photoluminescence (PL) spectrum of QDs is measured, and the PL-cross-section peak varying with wavelength is determined according to McCumber′s relationship between the absorption and emission. As the primary spectrum data of the QDs, the suggested cross-sections are useful for designing photoelectronic gain devices and sensors doped with QDs.