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Based on the detailed analysis of the microstructure on top surface of friction stir welded (FSW) joint of 2024 aluminum alloy by optical microscope,XRD and transmission electron microscope.SEM was employed to obtain the in-situ corrosion development of the joint.The results of microstructure observation show that many stringers are presented in the shoulder active zone (SAZ) due to the extrusion action of the tool shoulder.The grains and second phase particles are refined.The results of XRD reveal that the main precipitate in the alloy is Al2 CuMg (S phase).The results of in-situ observation of corrosion evolution indicate that the pitting corrosion initially originates in dissolving of the S phase.The density and degree of the pitting corrosion for the SAZ are slightly larger than those in other regions.The corrosion resistance property of the joint is deteriorative after FSW.