Abstract: The Rayleigh fractionation formula is an important method to study the isotope variation in water bodies under the influence of evaporation.The influencing factors on the coefficients of the Rayleigh fractionation formula are analyzed.A model based on the formula is solved for the slope of evaporation lines in water bodies.The influence of humidity and temperature on the slope is analyzed.The result shows that the slope of evaporation lines would reduce with the increase in humidity,and it would become larger with the increase in temperature.Using humidity and temperature data,the model is applied and validated on the middle reach region of the Yellow River Basin.The simulated slope of evaporation lines is compared with the surface water isotope data.