Crack detection is the basis of performance evaluation for steel structure. Metal magnetic memory testing method is introduced in this paper and investigations are conducted to verify its feasibility for detection and evaluation for steel structure. Firstly, static tensile experiment is conducted for two standard specimens to obtain the regulation between the coercivity and tensile force or strain. One of the specimens is base metal and the other one is with weld. In addition, the correlation between coercivity and state of fatigue cracking is obtained by conducting fatigue experiment of orthotropic steel deck. The investigation indicates that the value of coercivity is increased with the development of stress. And, the value of coercivity is more sensitive with the plastic strain. When visible fatigue cracks appears, the value of coercivity decreases with ratio of 10%. However, the value of it fluctuates for only about 2.5% before the appearance of visible fatigue cracks. The method is much more applicable for detection of visible fatigue cracks.