Abstract: The fatigue failure mode of aluminum alloy cruciform joint was carried out for the first time. By using the finite element analysis and fatigue test, the stress concentration factor s (SCF) of weld toe based on the traditional hot spot stress (HSS) and the equilibrium equivalent structural stress(EETS) were compared, the latter was mesh-insentive and shows high consistency. Besides, the influence of the Continuous member thickness of the joint, the penetration depth of the joint and the loading span on the EETS at the weld toe and the weld root are analyzed. And the effective traction stress was used to calculate the root cracking angle. It is found that, the equivalent structural stress at the weld toe keeps the constant, while the EETS at weld root changes with different continuous plate, the loading span and the penetration depth. The fatigue test shows that the root cracking angle is not 45°, which is consistent with the analytical prediction. The fatigue design of the cruciform joint needs to consider both the joint geometry and welding quality, which should meet the equivalent structural stress at the weld toe ( S_{s, toe}) is higher than the equivalent structural stress (S_{s, toe}) at the root of the weld.