Abstract: Fatigue life experiments with funneled-shape specimens were conducted to investigate the effect of stress amplitude on multi-axial high-cycle fatigue failure of 2A12-T4 aluminum alloy under tension-torsion proportional loading. Based on the experiment results, three kinds of life-prediction criteria, which were widely used for multi-axial high-cycle fatigue analysis, were discussed. The results showe that under the same von-Mises equivalent stress, fatigue life raised gradually when the ratio of stress amplitude in tension and torsion increased. Fatigue life of uniaxial tension is the shortest and that of pure torsion is the longest. The forecast results of equivalent stress criteria are either too dangerous, or too conservative. Stress invariant criteria cannot predict the impact of stress-amplitude ratio on multi-axial high-cycle fatigue failure. Critical-plane criteria has better prediction capability than others.