Abstract: In order to obtain the corrosion process and mechanism of joints by electromagnetic pulse(EMP) welded with aluminum to steel in neutral salt spray medium of 5%NaCl, shear test was performed and the shear fracture morphologies were analyzed by scanning electron microscopy. The results showed that the shear strength of the welded joint decreased from 74 MPa to 33 MPa after 3 days in the neutral salt spray corrosion, which was 44.6% of the original shear strength, and the weld failed completely after 7 days in corrosion. At the periphery of the weld, particle flow impacted the oxides at the surface of the aluminum plate to form granular corrosion NaAlO₂, and FeAl₃ at the weld was broken, exposing the aluminum to be rapidly corroded into Al(OH)₃. Where the impact pits at the surface of the aluminum plate and the embedded aluminum metal at the surface of the steel plate was the place first to be corroded, NaCl liquids accumulated and flowed under the metal surface for corrosion, and expanded along the direction of corrosion pits interconnection, and then extended to the joint zone where FeAl₃ phase existed in the weld. When the oxide or the weld was corroded by NaCl liquids and lifted and broken, it would be deeply corroded towards the aluminum matrix, and formed many and deep gullies or pits, which became the main corrosion mechanism of rapid joint failure.