Abstract: The existing initial model of gas entrainment is based on vertically downward small branch pipe entrainment, which is not applicable to the analy-sis of gas entrainment of large branch pipe. Based on the visualization experiment, the initiation point of gas entrainment for large and small vertical T pipes was studied and analyzed in this paper. The diameter ratios of branch pipes and main pipes are 0.625 and 0.1. The plexiglass was used as the experimental pipe material, and air and water as the experimental working medium. The experimental results show that there are vortices at the entrainment initiation of large and small branch pipes, and the vortices are roughly same. However, under the condition of large branch pipes, gas chambers will appear, and the entrainment of gas into the branch pipes will also billow under the gas chambers. In addition, under the same liquid phase conversion speed, the entrainment initiation liquid level of large branch pipes will be higher than that of small branch pipes.