Abstract: The liquid phase diffusion coefficient is an important basic data for studying mass transfer process, calculating mass transfer rate, and conducting chemical engineering design and development. Given the limitations of the existing methods of measuring liquid phase diffusion coefficient in terms of measurement accuracy and scope of application, a new method based on liquid-core cylindrical lens to rapidly measure the liquid phase diffusion coefficient by equivalent concentration thin layer move was proposed. By taking liquid-core cylindrical lens with spatial concentration measurement capability as diffusion cell and imaging element, the said method only needed to note down the time-dependent variation of a thin layer with a certain fixed concentration (fixed image width) in the experimental images, and the liquid phase diffusion coefficient could be calculated based on Fick's second law. At room temperature (25 ℃), the diffusion coefficient of potassium chloride aqueous solution was measured as 1.804 9×10⁻⁵ cm²/s by using this method. Meanwhile, the influence of selection of thin layers with different concentrations on the measurement results was studied. It was found that the calculated liquid phase diffusion coefficient was relatively stable when the concentration of the selected thin layers was less than 0.16 mol/L, and increased with the increasing concentration of the selected thin layer when the concentration was equal or greater than 0.17 mol/L. When the diffusion coefficient was measured by this method, thin layers of multiple concentration could be selected simultaneously in one experiment to calculate the liquid phase diffusion coefficient, which had the characteristics of simple experimental operation, short measurement time and accurate measurement results.