Abstract: Laser powder bed fusion (LPBF) have been frequently used to fabricate complex-shaped, dense, and high-performance oxide eutectic ceramics. However, the ceramic parts printed by LPBF still contain many pores that originate from bubbles in molten pool. These pores can seriously impair the mechanical properties of the ceramics. And the underlying mechanisms governing bubble behaviors in molten oxide materials are not yet clear. In this work, using synchrotron high-speed X-ray imaging technique, the formation, growth, motion behavior, morphological evolution of bubbles, and the relationship between the instability of melt flow and bubble rupture during LPBF of mechanically Al2O3-Y2O3 power were fully elucidated. We revealed that the bubbles mainly stem from intervals within the powder bed, and summarized three growth mechanisms, including float up pressure balance, gas channel and bubble coalescence. Additionally, melt flow oscillations induced by the bursting of large bubbles can lead to instability within local molten pool. These findings deepened understanding of the bubble evolution and can provide guidance for pore elimination in eutectic ceramics during LPBF.