Abstract: According to the metal magnetic memory(MMM) testing of 15CrMoR Y-slit type cracking after welding,the paper studied the magnetic memory characteristics of welding cold crack and got the law of magnetic memory signal in cold crack initiation and propagation process.Due to the complexity of stress concentration effect,firstly the wavelet transform was used to reduce the noise of magnetic memory signal and then the piecewise analysis method of maximum K was used to process and analyze the experiment results in the paper.The result shows that the MMM technology can locate the cold crack initiation and identify the propagation direction of crack by the change of magnetic memory signals.