Abstract: The rate coefficient of chemical reaction between hydrogen and lithium atomic vapor is an important parameter for a lithium isotope separation process based on laser induced photochemical reaction. In this paper, a method of measuring the rate coefficient was described. A tunable semiconductor laser was used as optical source. The relative change of the lithium atomic vapor density was achieved by the laser absorption spectroscopy measurements. Under the typical conditions of a photochemical lithium isotope separation process the measured result of rate coefficient is about 9.0×10^-22 m³/s. This value is of great significance for the determination of parameters such as laser illuminating time and density of hydrogen in a lithium isotope separation facility.