Theoretical analysis of D 1 lines of most stable alkali atoms in velocity selective optical pumping (VSOP) spectroscopy is presented by using the approximation of a single cycle of optical pumping in a four-level system. VSOP spectroscopy is clearly compared with polarization spectroscopy and then it is shown that the results obtained for polarization spectroscopy can be applied to the theory in VSOP spectroscopy. The relative signal intensities of Na D 1 line in polarization spectroscopy have been theoretically calculated and experimentally confirmed to be in good agreement with those in VSOP spectroscopy. The first-order theory in VSOP spectroscopy can be applied to the cases of the D 1 lines of the other alkali atoms. It is expected that the theoretical analysis can be useful for future experiments of the Doppler-free D lines in the alkali atoms.