Salt fog corrosion test was employed to observe the corrosion behavior of welded joint of 7N01-T4 aluminum alloy of high-speed train. Tensile strength and hardness change after corrosion were tested and micro morphology was observed by metalloscope and LCSM. The results indicated that HAZ was the most sensitive corrosion zone in welded joint of 7N01-T4 in chlorinated moisture environment. Influenced by the heat-input of welding, the grains of HAZ were coarsening and inhomogeneous. A corrosion cell between welding seam with high Mg content and HAZ with high Zn content formed. HAZ corroded preferentially as anode, and the dissolving of zinc-rich phase in grain boundary resulted in the extending of corrosion crack along grain boundaries.