Abstract: Plastic material flow is an important factor influenceing the surface shaping and mechanical property of joint during the friction stir spot welding process.In this paper, the plastic material flow behavior was investigated by lamella plates made up with thin copper foils and aluminum plates.The results show that, in the cross section of the joint, the plastic material in the both sides of the keyhole was equal wide in general.From top to bottom, the width of deformation decreased gradually.The plastic material at the top of joint was mainly influenced by the tool shoulder.Under the driving friction force of the shoulder and the shear between the shoulder and material, plasticmaterial flowed around the rotating pin, the trend of flow decreased with the increasing of distance to the shoulder.Under the action of screwed pin, the plastic material moved in a spiral flow around the rotating pin.When the plastic material flowed to the bottom of joint, the plasticmaterial moved upwards and outwards around the pin under the block of the backplane at the bottom and the extrusion force of un-plastic material, then plastic material that arrived the top moved to the bottom under the effect of shoulder, thus formed a cycle.