Abstract: The 5A05 aluminum sheet with a thickness of 4mm were welded under pre-tension stresses. Based on the measurement of the longitudinal warp distortion of weldments, the influence of pre-tension on the welding distortion of aluminum alloy was investigated. The experimental results showed that welding under pre-tension stresses could reduce the longitudinal warp distortion near to the welds of thin aluminum alloy plates effectively, and the greater the pre-tension stresses at the range of elasticity, the smaller longitudinal warp distortion. It was considered that pre-tension stresses enlarged the plastic distortion region, strengthened the resistance to plastic tensile distortion, and consequently decreased the longitudinal warp distortion of weldments.