Abstract: A 2D model for the disappearance of pre-welding interface and formation of FSW weld during friction stir welding (FSW) process was established. The forming process of friction stir weld from the pre-welding interface under the combined effects of welding heat,welding force and the stirring was described, and the friction stir weld can be divided into the grain growth zone,interface oxidizing zone,the pre-welding interface disappeared zone,plastic metal flow zone and "S" line zone. Meanwhile,the distribution of the pre-welding interface inside the weld was also identified in this paper. The model verified and explained the grain growth zone, "S" line and the pre-welding interface disappeared zone. The results showed that the prewelding interface was in metallurgical fusion state in the front of retreating side,and it twisted into the weld shaping "S" line under the stirring of the tool pin.