Abstract: The mechanism of the vane-type bubble separator developed for Thorium Molten Salt Reactor was investigated experimentally and numerically. The experimental phenomena and flow field obtained by the CFD software of Fluent were compared to analyze the process of the bubble coalescence and merging into a steady air core. It is demonstrated that, a rotational water flow with high radial pressure gradient forms after flowing through the swirl vanes of the separator, and that the bubbles in the flow moving to the center of the separator is due to that the centripetal force provided by the radial pressure gradient is greater than the centrifugal force generated by rotation. The bubbles will coalesce and merge to form a steady air core due to the large radial pressure gradient and low relative velocity in the central area of the separator, leading to the continuous separation of bubbles from the water flow.