The model of distribution of leakage magnetic field generated by remanence of stress concentration zone in ferromagnetic component is investigated. According to theory that the zero-crossing point of normal magnetic field is the maximum of the gradient

normal magnetic field gradient is regarded as characteristic value is applied to judge stress concentration degree. The structure parameters of piezoelectric ceramic cantilever which is used to drive probe vibration is designed

and use vibration frequency as center frequency of signal

design the secondorder narrow-band filter and technique of extracting the amplitude of alternating signal is investigated

and use orthogonal lock-in-amplifier technology to separate weak normal magnetic field gradient component from strong noise background. The circular section specimen of A3 steel verify the relationship between strength of magnetic memory signal and stress concentration coefficient.