Abstract: Residual stress relaxation behaviors of A7N01 aluminum alloy structures of underframes were investigated. The A7N01 aluminum alloy welded joints was treated by pre tension treatment and surface shot peening technology for obtaining the preset residual stress. Then the fatigue behaviors of different welded joints were investigated. The results showed that the residual stress had undergone larger relaxation after 1×10⁵ cyclic loading. The residual stress relaxation extent had undergone gently after 2×10⁵ cyclic loading and finally became to stable with the value at 20~40 MPa. The real cyclic stress ratio was changed different preset residual stress treatment. When R≥0, the fatigue cycles decreased with the increase of stress ration. The residual stress decreased the origin period of cracks and changed the propagation velocity of cracks.