In order to study the temperature characteristics, strain distribution and material flow law during differential bobbin tool friction stir welding（DBT-FSW）,an all-heat-force coupling model is established for the 5 mm thick AZ91 magnesium alloy plate based on deform-3D software to conduct numerical simulation of DBT-FSW and analyze the similarities and differences with bobbin tool friction stir welding. The results show that the temperature field of DBT-FSW in the direction of thickness shows an approximate "vase" distribution,while the temperature in the affected area of the upper shoulder is lower than that of the lower shoulder. In the horizontal direction,the temperature of DBT-FSW is higher on the retreating side than on the advancing side. As for the strain field distribution,the equivalent strain on the advancing side is higher,which is contrary to the law of temperature field distribution. In terms of material flow characteristics,the advancing side of DBT-FSW has more active material flow. In the direction of thickness of DBT-FSW,the influence area of the shaft shoulder on the side with higher rotation speed is larger,and the material in the middle part moves towards the side with higher rotation speed.