Metastable austenitic 304 stainless steel is widely used in the field of nuclear power, but it is prone to fatigue damage and affects the service life; thus, it is crucial to assess the fatigue damage during service via nondestructive testing meansmethods. The correlations among the microstructure, Vickers hardness, and saturation magnetization of 304 stainless steel under different fatigue cycles were investigated via X-ray diffraction, electron backscatter diffraction, hardness, and magnetic property testing. The experimental results show that with increasing number of fatigue cycles, 304 stainless steel transforms from the austenite phase to the martensite phase, and the martensite phase content continues to increase; the dislocation density increases rapidly during the fatigue process from 0 to 1 cycle and remains relatively constant thereafter. The martensite content and number of dislocations affect the Vickers hardness of 304 stainless steel during 0 to 1 cycle of the fatigue process; as the number of fatigue cycles further increases, the increase in the Vickers hardness is related to only the increase in the martensite content, and a linear relationship exists between them. The saturation magnetization of 304 stainless steel is associated with the martensite content in the steel, and there is also a linear relationship between the two.