In this investigation, in order to research the influence of magnetic field frequency on microstructure and properties of magnesium alloy welded joint, the AZ31 magnesium alloy plates which was 5mm were welded by GTAW under longitudinal magnetic field. The tensile test, hardness test and SEM were taken place to analyze the properties and microstructure of welded joint under different magentic field frequency. The acting mechanism of magentic field on welded joint was studied. The results show that: External magnetic field can improve the properties of magnesium alloy welded joint. The optimal mechanical property will be obtained when Im=2, f=20Hz, which the ultimate tensile strength is 231Mpa and the micro-hardness is 76.2HV.The magnetic field can produce electromagnetic stirring, which can refine crystal grain through breaking the pine-tree crystals with proper magnetic field current and frequency. The properties of welded joint will been improved by these fine crystal grain.