The corrosion behavior and mechanism of friction stir weld (FSW) joint of 2219 aluminum alloy was investigated by in-situ corrosion test, static mass loss test and immersion test. The results show that the corrosion rate of the weld is less than that of the parent material and the corrosion resistance of the weld improves greatly. The local pitting corrosion initially originates in dissolving of the adjacent matrix around the second phases and the second phase particles act as cathode. The intergranular corrosion happens in the weld nugget zone (WNZ) and thermo-mechanically affected zone (TMAZ) after in-situ corrosion test for 2 h and pitting corrosion occurs in base materials. The homogeneous second phase particles and fine equiaxed grains in the WNZ are the main reasons for the high resistance of exfoliation corrosion.