Abstract: The two-dimensional model of bubble flow in molten pool of vacuum electron beam welding on AZ91D magnesium alloy is developed based on the conservation of mass and momentum and the system of gas-liquid two-phase-flow.The gas distribution and velocity distribution of bubble flow in molten pool are simulated to analyze the formation and distribution of cavity defects.The results show that the possibility of gas escape in fully penetrated molten pool is much greater than non-penetrated molten pool.This also indicates that the probability of cavity defects in fully penetrated weld is lower than non-penetrated weld to some extent.The formation of cavity defects is closely related to the flow pattern and flow characteristics of the bubble flow in deep penetration molten pool.Higher liquid flow rate is more conductive to the escape of gas in molten metal, so that the final porosity in that weld is low.