The dependence of electron spin relaxation on the momentum relaxation and carrier density are investigated based on the spin-orbit coupling and D'yakonov-Perel relaxation mechanism. Experimental results obtained by using femtosecond pump-probe technique in AlGaAs/GaAs multiple quantum wells at room temperature show that the spin relaxation time increases from 58 to 82ps with the carrier density increases from 1×1017 to 1×1018cm-3, consistent with the theoretical prediction. This results reveals that with the increase of the carrier density, the spin-orbit interaction reduces due to more frequent momentum scattering, sothat the spin relaxation time prolongs.