The isotope effects on the absorption spectrum of Li in solid hydrogens at T = 5 K and different pressures are extensively studied. At a lower pressure, the difference in Li atomic absorption spectra between the Li/D-2 and the Li/H-2 systems is significant, while becomes less remarkable with the increase in pressure. The zero-point motion from H-2 molecules influences the Li absorption spectrum in both a direct and an indirect way. Although the direct and indirect effects on the absorption spectrum from the quantum motion cancel partly, eventually the indirect effect dominates the variations of the Li absorption spectrum.