Abstract: One of the welding problems for magnesium alloy is the softening of the welded joints. Deep cryogenic treatment is put forward to strengthen joints of AZ31 magnesium alloy welded by TIG. The welding experiment and deep cryogenic treatment test for 8h at -160℃ are carried out. The tensile strength of the joints treated by deep cryogenic is tested as well. The microstructure of the deep cryogenic joints of magnesium alloy has been examined with the optical microscope,the scan electron microscope,the transmission electron microscope and the X-ray diffraction. The mechanism of deep cryogenic strengthening joint of AZ31 Magnesium alloy has been analyzed. The results show that tensile strength are increased from 212.4 MPa to 246.6 MPa and the strength coefficient is increased from 83.7% to 97.1% by deep cryogenic treatment. Moreover,the subgrain structure is formed in the welded joints of AZ31B by deep cryogenic treatment. The Mg17Al12 particles precipitate and improve the continuity of base material. The number of second phase particles increases,which refines the microstructure of welded joints by TIG of AZ31B magnesium alloy and strengthen mechanism. The dislocation of deep cryogenic joints changes into the dislocation loop and twin crystal is formed as well. The orientation of grain crystal of welded joints of AZ31B has change,resulting in the grain rotation after deep cryogenic treatment.