In this paper, collisional quenching rate for Li(2P) atoms with argon molecules is studied in photochemical isotope separations. In the weak laser irradiation, the absorption spectra and fluorescence emission spectra of lithium atom vapor is measured by changing the argon pressure. The quenching rate constant of (12.29±0.92)×10-18 m3·s-1 for Li(2P) atoms quenched with argon molecules have been determined. This value is much smaller than the spontaneous emission rate of Li(2P) atoms. The experimental result shows that in photochemical lithium isotope separation collisional quenching effect on the separation selectivity is very small and can be neglected.