A light beam coming from a heterodyne light source and is divided into three parts by two beam-splitters. Two parts are guided to be incident on the thin metal film on a surface plasma resonance (SPR) probe from two other sides. Two groups of multiple reflected beams occur in the thin metal film, they pass through their corresponding analyzer and lens, and are detected by the associated photodetectors. So we have two test signals. On the other hand, the other part passes through an analyzer and enters a photodetector. The intensity measured by the photodetector is the reference signal. These two test signals and the reference signal are send to a phse meter, the phase differences between the test signals and the reference signal can be obtained. Substituting these data into the special equations derived from Fresnel's equations and the principle of multiple beam interference, the complex refractive index and the thickness of the thin metal film of the probe can be estimated. Because of its common path optical configuration and its heterodyne interferometric phase measurement, this method has many merits, such as, high stable against the surrounding vibration, high resolution, real time operation. In addition, four samples are tested to show the validity of this method.