In order to study the effect of laser shock processing (LSP) on the stress corrosion cracking (SCC) of welded magnesium alloy, tungsten inert-gas (TIG) welded AZ31B magnesium alloy sheets surface is processed using NdYAG laser with a wavelength of 1064 nm, a pulse width of 15 ns, a pulse energy of 4 J and a spot diameter of 3 mm. At room temperature, the SCC susceptibility of the weldment samples with and without laser treatment is assessed by three points loading method in deionized water. The microstructure of specimens by LSP and fracture surfaces are analyzed by optical microscopy (OM), transmission electron microscope (TEM) and scanning electron microscope (SEM), respectively. The results show that a nanostructured surface layer can be produced on welded AZ31B Mg alloys by using optimized laser parameters and surface nano grain size is about 35 nm. The surface residual stress is converted from tensile stress (60 MPa) to compressive stress (-125 MPa). LSP can reduce the corrosion trend of the weldment samples because corrosion potential increases by 88 mV and corrosion current decreases by 73.4% as compared to the samples without LSP. SCC appeares on samples without laser treatment after 192 h of immersion, however, no cracks is observed on samples treated by laser after 10 months, which shows that LSP can obviously improve the resistance to stress corrosion cracking of TIG welded AZ31B magnesium alloy.