Four styles of spectral line shape function and their transformation relation were investigated theoretically. The photon number and energy distribution as functions of frequency and wavelength resulting from Doppler broadening were investigated, and the maximum intensity and FWHM(Full width at half maximum intensity) were obtained. The results show that the photon number distribution as function of frequency is almost the same as the energy distribution as function of frequency, while the photon number distribution as function of wavelength is almost the same as the energy distribution as function of wavelength. The method has been presented by analyzing the Doppler spectral line shape function, with which the ratio of density between two spectral lines can be obtained according to the ratio of maximum value of two spectral lines.