Abstract: The technology of laser absorption spectroscopy was used for lithium isotope ratio measurement. A tunable external cavity diode laser, operating at a fixed center wavelength, was used as an optical source. The measurement and control of lithium metal evaporation temperature was achieved by the PID temperature controller. The using of laser oblique incidence eliminates the influence of etalon effect in the optical path. The absorption spectra of ⁶Li and ⁷Li near 671 nm were recorded at six different cavity temperatures. The ⁶Li/⁷Li isotope ratio was retrieved from the integrated absorbance of ⁶LiD₁ and ⁷LiD₂. Experiment results show that the best accuracy of lithium isotope ratio is 2.5%.