In order to study the behavior and mechanism of the bubble breakup in Advanced-Flow Reactor (AFR), visualization experimental study was carried out in a single Corning G1 module. The results show that the breakup behavior in AFR has a significant regularity at low Reynolds numbers (Re<1000), which is closely related to the capillary number Ca and the virtual length l0 of bubbles, and the criterions is also obtained. Most of bubbles in arc-shaped junction divergence of AFR are breakup with permanent obstruction (POB), which could be divided into two stages: squeezing stage and rapid pinch-off stage. The modified equation obtained in this paper can roughly describe the change of the bubble neck at the squeezing stage of POB. The research results provide a theoretical basis for industrial application of AFR and the development of next-generation reactors.