A new technique for studying surface plasma is reported. With this technique, the phase and amplitude information of the reflection light can be detected as surface plasma is excited. Theory shows that the phase of P polarization can change from 180° to～-120° during the excitation of surface plasma. An optical heterodyne system for measuring phase and amplitude is set up. Not only phase and amplitude information can be detected at the same time, but also the sensitivity is better than that of the ART method. This new methot can be used for studying LB film and surface sensors.