Abstract: Double-pulsed metal inert-gas welding was used to weld AA7075-T651 aluminum alloy, and the welding samples was treated by solid solution treatment(480℃×50 min) and water quenching, then double-aging heat treatment(80℃×24 h)+(120℃×24 h). The effects of post weld heat treatment (PWHT) on microstructure and mechanical properties of welded joints were investigated by means of optical microscopy(OM), scanning electron microscopy(SEM), X-ray diffraction analysis, tensile test and micro-hardness measurement. The results shows that the grains are changed from dendrite to isometric crystal, and the non-equilibrium second phase is dissolved,the grain boundary becomes narrower and finer after post-weld heat treatment, and the microstructure characteristics of the welded joint are improved obviously. After heat treatment, the tensile strength of the welded joint increases from 342.5 MPa to 490 MPa, and the strength coefficient of the welded joint is up to 0.872. The micro-hardness of the weld softening zone is greatly improved, and the mechanical properties of the welded joint are significantly improved.