It has been a long time that in pulsed eddy current (PEC)testing the permeability of ferro-magnetic metallic materials is considered to be constant.However,by now there is insufficient evidence for the rationality of this simplification and its influence.This unresolved problem was studied in this work.Based on finite element method,the model of a PEC probe placed on a Q235 steel plate was estab-lished.Then,the space distribution and time variation of the steel plate's permeability and their influ-ences on the probe signal were studied.Also,an experiment validation of the simulation was conducted. The results are as follows:(i)the working range of transient magnetic field in the steel plate varies with the amplitude of excitation current,and when the current is increased the magnetic field probably ex-ceeds the initial magnetization region and reaches the Rayleigh region even the steep region;(ii)due to the nonlinear permeability the two induced voltage signals acquired during the high level and low level of the excitation square wave were not odd-harmonic symmetric,and the late signal of the high level was larger than that of the low level;(iii)in shallow layer of the steel plate the permeability varies signifi-cantly and thus cannot be simplified as constant,but in deep layer the permeability can be regarded as constant due to its little variation.%长期以来，脉冲涡流检测中铁磁材料的磁导率多被视为常数，但这种简化的合理性及其影响尚缺乏充分的证明。本文基于有限元法，建立了探头置于 Q235钢板上方的脉冲涡流检测模型，研究了钢板磁导率的空间分布和时间变化规律及其对探头信号的影响，并进行了实验验证。结果表明：钢板中瞬态磁场的工作点与激励电流幅值有关，增大激励电流，磁场可能超出磁化曲线的起始磁化区而进入到瑞利区甚至陡峭区；由于磁导率非线性的影响，激励方波高、低电平段的感应电压信号不成奇谐对称，高电平段的晚期感应电压比低电平段的大；钢板浅层磁导率的变化显著，不能简化为常数，而深层的磁导率变化很小，可视为常数。
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