Abstract: During the work of photo-chemical lithium isotope separation, the density of lithium vapor needs real-time monitoring. The laser power, reaction temperature, pressure and time parameters of separation were optimized by analysis of isotope ratio. The lithium isotope ratio under non-weak absorption conditions can be quickly and accurately measured by absorption spectroscopy. Under weak absorption conditions, isotope ratio can not be accurately measured because of round-off error and resolving limit of the oscilloscope. In order to solve the problems in lithium isotope ratio measurements, a high resolution and precision signal acquisition method based on differential circuit was proposed. The ratio of natural materials was measured by using this method, and the result is 12.37±0.56. This method can be commonly used in isotope ratio measurement by absorption spectroscopy and can also be applied in other similar separation work.