Abstract: As an example of experimental material, AZ31B magnesium alloy was butt welded with filling wire of similar composition by TIG welding method.The fusion welded joints attained were put in a vacuum furnace to be thermo plastically extruded at constant temperature of 400℃ and then the fatigue properties of joints were tested at room temperature.With the help of scanning electronic microscopy, optical metallographic microscope and energy spectrum analyzer etc., the experimental specimen were analyzed to explore the way and micro-mechanism of improving fatigue properties with thermo plastic extrusion for fusion welded joint of magnesium alloy.The results show that with thermo plastic extrusion, the fusion welded joint fatigue life of magnesium alloy can be improved effectively, this mechanism is chiefly due to beneficial effecting for oxide to be broken and micro-crack to be closed or healed with constant temperature extrusion at 400℃, also the coarse α grain in fusion zone may be promoted to transform into sub grain boundary, therefore a refining microstructure can be attained.Meanwhile it causes weld microstructure to reconstruct by way of dynamic re-crystallization and changing network-like distribution of β-Mg17Al12 precipitate on α-Mg grain boundary of weld microstructure and making it solid solve to re-precipitate in way of dispersed distribution inside α grain.