Abstract: Aiming at the detection of surface defect of metal plate,the low-frequency electromagnetic detection principle and detection methods were studied,and low-frequency electromagnetic detecting method for metal plate defect was proposed.A low frequency electromagnetic detection model and three-dimensional simulation was established.The effect of excitation signal intensity and excitation frequency on leakage magnetic field magnetic induction intensity was analyzed.Influence of different sizes of defects and lift-off on the leakage field magnetic induction intensity was also analyzed.Excitation and detection coil was produced,and experimental study of low frequency electromagnetic detection was performed.Experimental results showed that,when excitation frequency was 20Hz,the detected amplitude of the signal was maximum.With increasing depth of the defect,the amplitude of detection signal increased.And with the increasing in lift-off of detection coil,detection signal amplitude decreased.