Some parts of aluminum alloy body of China Standardized EMU are welded by friction stir welding. The front bottom plate adopts bobbin friction stir welding process. During the production of the front bottom plate, the first pass rate of welding（phased array ultrasonic testing） is less than 50%, in view of the strict requirement on quality, it seriously affects the welding quality and delivery schedule. Aiming at the above problems, the welding defects were studied. ABAQUS was used to simulate the formation mechanism of cavity defects, cavity defects can be avoided with appropriate parameters. The normal and abnormal fracture zigzag lines were studied by OM and SEM, the origin and formation mechanism of the zigzage lines in the two kinds of defects were discussed. Through the improvement of the tool profile, the original triangular prism shaped agitator needle is changed into a positive and negative double helix shaped agitator needle, the research of welding technology has been improved systematically, the first pass rate of welding reaches 100%.