Abstract: Based on the mechanism of non-destructive testing of metal magnetic memory, the magnetic domain structure of ferromagnetic stress concentration has magnetostrictive directional and irreversible reorientation. Designed “in situ” fatigue test and used Bitter powder pattern method for annealing 16MnR steel domain structure observation and compared the different effects of fatigue cycles to the magnetic domain. The results showed that the domain of whole research region was main 180 degrees domain structure, and branches domain and a small amount of “tetragonum” type domain. As the fatigue cycle changes, the domain wall of selected grain was easy to occur about 45 degrees or 90 degrees rotation, leading to the domain structure changed among lamellar domain-branch domain-“tetragonum” domain types.