Abstract: The physical meaning of parameters in isotopic fractionation model, was discussed, and the sensitivity of enriched isotopic components of liquid to free air relative humidity was emphatically studied. Some conclusions have been reached: ①The free air relative humidity determines the dominant range of diffusion fractionation factor, αdiff, and isotopic components of free air in the equation of evaporation fractionation factor, α. The smaller the relative humidity, the smaller the impact of free air components on α, and the larger the influence of αdiff on α, and vice versa. ②The enrichment degree of hydrogen and oxygen stable isotopes in the process of evaporation is negatively related to the relative humidity of free air. ③The sensitivity of enriched liquid components to free air relative humidity is an exponential function. The sensitivity simulation shows that the sensitivity is smaller than 0.0002 when relative humidity is smaller than 80% under Boaz’s experiment condition, which indicates that the dynamics of air humidity in the range from 0% to 80% can barely affect the isotopic components of liquid water in the end of the evaporation stage, while the sensitivity exponentially increases with relative humidity when it is larger than 80%, which indicates that experiments with a free air relative humidity larger than 80% should measure humidity accurately.