The commercial pure zirconium samples were processed by surface mechanical attrition treatment (SMAT) to realize the surface nanocrystallization, and the compressive residual stress was induced at the same time. The microstructure was studied by PM and TEM after SMAT. The macroscopic residual stress at different depths from surface was evaluated by XRD after different cycles. The distribution characteristics of FWHM were investigated. Fitting the peaks from XRD by Voigt function, microstrain and dislocation density at different depths from top surface after different cycles were obtained. The results indicate that the biggest relaxation rate (about 25%) is found at initial stage, and the more cycles, the less residual stress relaxation. Under different cycles, with the increase of the depth from the top surface, the FWHM decreases rapidly, and then reaches to a steady state. The distribution characteristics of the microstrain and dislocation density at different depths from surface are similar to FWHM after different cycles. Along with the decrease of dislocation density, the residual stress increases gradually. The depth where the dislocation density value is about 2×1010 cm-2 is similar to that of the maximum compressive residual stress. The residual stress relaxation is achieved by the accumulation of plastic deformation in the process of dislocation motion under cyclic loading