The mathematic model between the phase distribution of object beam and the mutual diffusion coefficient of binary solution was established based on the one-dimensional and infinite model. A novel experimental method for measuring the mutual diffusion coefficient of binary liquid mixtures was proposed based the existing digital holographic interferometry. For the presented method

the mutual diffusion coefficient is determined directly through the phase distribution curve of object beam

which can obviously avoid the influence of ambient noise on the experimental result. The requirement of the optical condition in the lab was considerably reduced. The relative combined uncertainty of mutual diffusion coefficient is around 0.7%. The mutual diffusion coefficient of KCl aqueous solution at 0.33 mol/L and 25℃ was measured. The result is in good agreement with literature data

which proves that the new measuring principle is feasible and correct. With the new method and experimental apparatus

the mutual diffusion coefficient of methanol/cyclohexane binary mixture was measured and analyzed near the immiscible concentration region. Based on the experimental result

the Spinodal line

which is used to describe the boundary between the thermodynamic instable region and the metastable region

was predicted for methanol/cyclohexane binary system.