To detect the damage of inner and outer walls of metal plate members, a multichannel lowfrequency electromagnetic sensor designed by means of numerical simulation and experimental optimization. In this study, a threedimensional finite element simulation model of lowfrequency electromagnetic detection is formulated. In this way, the defect detection capability of circular coil placed tangentially over metal plate conductor can be studied. The comparative test is conducted to optimize the number of turns of the excitation coil, the inner diameter and the height of the detection coil, and other parameters, which affect the sensitivity of the sensor. The sensitivity and practicability of the sensor are evaluated by detecting artificial defects contained in the metal plate. Experimental results show that the multichannel lowfrequency electromagnetic sensor can effectively detect the defect of 304 stainless steel with 12 mm buried depth of 96 mm and 20# carbon steel with 12 mm buried depth of 8 mm.