Abstract: Tablet samples of 45# steel with U-type defects were tested under repeated loading and unloading for fatigue test and were analyzed by measuring the leakage magnetic field of the sample surface normal distribution.The effect of U-shaped defects on the magnetic memory signals was analyzed and the relationship between magnetic field values and the cycle at the fixed position was also analyzed.It was found that an inversion phenomenon of the magnetic signal appeared at 150,000 cycles times (about 30% final week cycle times) for the specimen.Comparison was made for the variation of the magnetic signal characteristic parameters Kmax and the material damage parameter D in damage mechanics model and it was found that the parameter D could be expressed by the Kmax,which might provide a means for using the metal magnetic memory testing technique to evaluate the extent of material damages.