An improved method for measuring the refractive index of a medium is presented. First, the light coming form a circular heterodyne source is incident on the test medium. And, the reflected light passes through an analyzer with the transmission axis at a moderate azimuth angle. Then, the phase variation of the reflected light is measured by the heterodyne interferometric technique. Finally, substituting this phase variation into the equations derived from Fresnel's equations and Jones calculus, we can obtain the refractive index of the test medium.