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A new method based on self-mixing interference (SMI) for refractive index measurement by means of simply frequency analysis is proposed. The optical phase shift is implemented by translating a transparent object through a motorized stage with a setting velocity. Then, the refractive index is determined by the frequency of SMI fringes. A relative repeatability of refractive index estimated in 1.0 x 10^-4 is achieved experimentally. This method has found that there exists an optimal velocity to achieve minimum error of refractive index measurement.