The invention discloses a device and a method for detecting damage to a composite by the aid of Lamb guide waves. The device comprises a composite plate, multiple PZT sensors, an arbitrary wave generator, a high voltage amplifier, a data acquisition unit and a computer, wherein the computer is used for acquiring and calculating exciting signals sensed by the PZT sensors and corresponding signals sent by the PZT sensors, range envelope is performed on Lamb response signals of the composite plate in a healthy state and Lamb response signals of the composite plate in a damage state, subtraction is performed, damage signals generated after the Lamb waves are reflected and scattered through a round hole (the round hole is equivalent to the damage position of the composite plate) are obtained and subjected to superposition, influence of random errors can be eliminated, the amplitude of the damage signals is increased, the accuracy and the reliability of a measuring result are greatly increased, and the deviation between the measured damage distance and the actual distance can be kept within 7%.