In order to improve the thickness measurement accuracy of metal plate, a new method based on phase detection thickness is proposed. Two modes of ultrasonic echo are generated by electromagnetic acoustic transducer (EMAT) at the edge of specimen, and super-heterodyne phase-sensitive detection technique is used to collect the phases of the ultrasonic, then the thickness information of the tested specimen is obtained accurately. Firstly, the finite element model of EMAT is built, and the time-domain characteristics of acoustic echo are analyzed. Secondly, the 0^∘ and 90^∘ phase detection on echo signal are detected by the super-heterodyne phase-sensitive detection system. Finally, the relationship between phase information and specimen thickness is analyzed. The results show that the detection method, which based on super-heterodyne phase-sensitive detector of edge mode conversion, can realize the detection of metal plate thickness, and the phase error range is controlled within 0.23%.