This　paper　focuses　on　engineering　application　of　metal　magnetic　memory　(MMM)　method.　High-cycle　fatigue　test　were　applied　to　a　large　number　of　defect-free　samples,　and　MMM　signals　were　captured　before　fatigue　fracturing.　A　concept　of　critical　magnetic　intensity　(Hp)　derivative　(dHp/dx)　of　cracking　was　proposed.　To　study　the　characteristics　of　Hp　and　its　derivative　(dHp/dx)　during　cracking,　fatigue　test　were　applied　to　a　total　number　of　50　pieces　of　Q235,　16MnR　and　base　metal　and　welded　specimens.　In　different　fatigue　periods,　MMM　signals　were　tested　and　microscopic　metallographic　were　observed,　so　as　to　quantitatively　analyze　Hp　and　dHp/dx　from　both　macro　and　micro　angles.　The　results　show　that　various　materials　differ　in　their　critical　dHp/dx　limit,　which　could　be　the　fatigue　crack　initiation　time.　It　thus　proves　that　MMM　has　special　efficiency　in　the　diagnosis　of　premature　defects　and　stress　concentration　of　metal　structures.