The surface gradient structure is widely obtained in engineering materials. After the surface strengthening technology treatment, the surface structure of the part shows a gradient distribution, which can improve the serviceability of the part. Reviews the influence of the gradient microstructure of the metal surface layer on the mechanical properties and crack growth. From the perspective of microstructure and mechanical properties, combined with the crack growth behavior in heterogeneous materials, the mutual influence law between microstructure and mechanical properties is considered and the factors affecting crack growth in the gradient structure are analyzed. After surface strengthening, the surface grain of the material is refined, and the grain size presents a gradient distribution along the depth direction, which changes the position of crack initiation, and at the same time hinders the crack propagation and prolongs the fatigue life of the part. The metallographic structure of the gradient structure is single from the surface. The transition from the structure to the multiphase structure of the core causes the surface hardness and residual compressive stress to gradually decrease in the depth direction. The surface compressive stress can weaken the action of external forces, thereby affecting the fatigue life. However, the microstructure and mechanical properties in the gradient structure are related to each other. For example, grains and dislocations can change the distribution of residual stress and also have an impact on the crack propagation path. How to quantitatively characterize their effects remains to be further studied.