Microfluidic technology based on microchannel two-phase flow has been widely used and precise control of the bubble or droplet size in the channel plays a crucial role in the design of microfluidic systems. In this paper, the bubble breakup behavior in Y-shaped microchannel is reconstructed based on the volume of fluid method (VOF), and the effects of bubble dimensionless size (1.2 ~ 2.7), outlet flow ratio (1 ~ 4) and main channel Reynolds number (100 ~ 600) on the bubble breakup behavior are systematically investigated. The bubble asymmetric breakup process was found to be divided into three stages: extension stage, squeeze and rapid pinch-off. In the case of small initial bubble size or relatively high outlet flow rate, the bubble does not break only through the extension stage and the squeezing stage. Four flow patterns of bubble breakup are further revealed for different sizes and outlet flow ratios: tunnel-tunnel breakup, obstruction-obstruction breakup, tunnel-obstruction breakup and non-breakup. With the increase of outlet flow ratio, the breakup process of the bubble gradually becomes asymmetrical, and the flow pattern shifts along the tunnel-tunnel breakup/obstruction-obstruction breakup, gradually to tunnel-obstruction breakup and non-breakup direction. On this basis, the critical flow ratio of bubble breakup and the variation of daughter bubble volume ratio with outlet flow ratio were obtained for different Reynolds number and initial bubble size, and the corresponding criterion correlation equation was refined, which can provide theoretical guidance for accurate regulation of daughter bubble size after breakup.