The active diagnoses technology based on the Lamb wave and piezoelectric sensor is an effective method for the composite structural health monitoring. The characteristics of the Lamb wave propagation is complex. And it is difficult to distinguish the different modal separately for the frequency dispersion and modal aliasing. This paper proposes a method of using the double piezoelectric wafer to extract the single modal Lamb wave. It can effectively solve the signal aliasing caused by modal mixing. Because this method is based on the physical properties of the sensors and does not depend on the specific structure or sensors layout features, so this method has better adaptability to the project. The validity of the proposed method has been verified by the experiment of extraction single mode of the Lamb wave in the carbon fiber composite plate.