<p>Symmetric tension–compression cyclic loading tests have been conducted on extruded magnesium alloy AZ31 under different strain amplitudes in order to investigate the cyclic hardening behavior of magnesium alloy AZ31 under such loading and to explore the relationship between twinning deformation and cyclic life. Statistical analysis was performed on the twin nucleation number and twin thickness. The results indicated that, at a strain amplitude of 0.8%, the cyclic hardening curve develops linearly and steadily. At amplitudes less than 0.8%, the cyclic hardening rate initially is slow but then accelerates, while at amplitudes greater than 0.8%, the cyclic hardening rate initially accelerates but then slows. The statistical analysis of the twin characteristics revealed an increase in twin nucleation with higher strain amplitudes, leading to a decrease in fatigue life. Twin thickness remained consistent across different strain amplitudes, indicating a state of residual twin saturation. At the same time, the statistical relationship between twin thickness and cycle life shows a weak correlation. The study’s results can provide basic experimental data to effectively characterize the correlation between twin deformation and fatigue life.</p>