Numerous fatigue crack mechanism models have been proposed based on an in-depth study of material fatigue mechanisms and engineering requirements. However, due to many of the parameters in these models being difficult to determine, their application to engineering is limited. The fatigue crack of the steel structure was calculated based on the improved McEvily model. To begin, based on the theory of linear elastic fracture mechanics, some parameters of the McEvily fatigue crack growth model were deduced and determined by using more reasonable assumptions and empirical formulas. Second, the effectiveness of the improved McEvily fatigue crack growth model was proven by comparison to the results of the improved model with the classical Paris model. Finally, the improved McEvily model was applied to practical engineering, and the typical fatigue crack of steel structure was selected and compared with the results of the Paris model and nominal stress method to verify its feasibility in engineering. The results reveal that the application conditions of the improved McEvily model can be extended from laboratory conditions to practical engineering, and its accuracy is better than that of the Paris model, which can well evaluate the fatigue crack life of steel structures.