Most of the damage of metal materials originates from the surface, so surface treatment plays a vital role in improving the service life of metal materials. The traditional materials with uniform microstructure show a tradeoff in strength and ductility. Coarse-grained materials usually maintain considerable ductility, but the strength is relatively low; while the strength of ultra-fine-grained is elevated, the ductility is dramatically decreased. Materials with gradient nano-grained (GNG) structure, where grain size changes gradually from the treated surface to the core, are promising for overcoming the strength-ductility contradiction. Moreover, the GNG materials also possess other outstanding mechanical properties, such as enhanced wear resistance, improved fatigue life and fracture resistance. In this paper, the GNG copper was prepared using the SMAT technique, and uniaxial tensile tests were performed on both GNG and coarse-grained copper. The test results showed that the SMAT greatly improves the yield strength, without compromising too much ductility. Furthermore, Cyclic deformation tests show that the number of cycles of SMAT samples to failure is significantly higher than that of as-received samples, and the ratcheting strain of SMAT samples is significantly lower than that of as-received ones. In other words, the SMAT effectively inhibits the ratcheting strain of pure copper and enhances its fatigue life. Microscopic characterization was carried out, including electron backscatter diffraction (EBSD) analysis and X-ray diffraction (XRD) analysis. It is found that the average grain size of the material after SMAT decreases, while the average dislocation density increases. In addition, the grain size and total dislocation density show a spatial gradient from the surface to the core. In the end, based on a modified version of the conventional theory of mechanism-based strain gradient plasticity (CMSG), the mechanical responses of pure copper before and after SMAT are simulated and compared with the results of the tension-compression cyclic test. The study found that the constitutive model can well describe the uniaxial tensile response of the material after SMAT, and can qualitatively capture the inhibition effect of the SMAT technique on the ratcheting strain evolution.