Diffusivity of redox shuttle additive in solvent plays an important role in the design of Li-ion battery. As redox shuttle additive, 10-methylphenothiazine (MPT) can give good overcharge protection in LiFePO4-based Li-ion cells, therefore diffusion coefficient of MPT in solvent needs to be known. A different data processing method based on optical digital interferometry is put forward to determine diffusion coefficient in this work. This method is verified by KCl aqueous solution and the relative standard uncertainty of diffusion coefficient is about 2.3%. The diffusion coefficients of MPT in dimethyl carbonate (DMC) from 288.15K to 313.15K at several concentrations are measured and the corresponding calculation equation of diffusion coefficient is presented.