With the rapid development of China’s economy, and with metal plates being widely used in aerospace, the national defence industry, shipbuilding industry, high-speed railway and so on, the domestic demand for metal plates is increasing. However, in sheet metal processing and production processes, due to the influence of the environment, there will inevitably be a variety of different types of defects. As a result, it is of great significance to choose a reliable method to carry out a large area detection of the metal plate. The electromagnetic ultrasonic guided wave transducer, because of possessing the advantages of both the Electromagnetic Ultrasonic Transducer (EMAT) and the ultrasonic guided wave, which is namely non-contact, large detection area, high sensitivity, small attenuation, and long distance transmission, is very suitable for non-destructive testing of large areas of sheet metal. However, its energy-change efficiency is not high. In order to improve the non-destructive testing of the electromagnetic ultrasonic Lamb wave transducer, this paper studies the optimisation method of the Lamb wave transducer in the detection of metal plate, carries out simulation calculations of the Lamb wave transducer after optimisation, and applies the optimised Lamb wave transducer in the detection of sheet metal defects. Simulation and experimental results show that for the Lamb wave, in the face of defect mode conversion, as the defect depth increases, the received echo signal amplitude increases, which characterises the defect echo size.