Pressurized dissolved air flotation is an efficient process commonly used in wastewater oil removal, and the effect of bubble size and distribution on oil removal efficiency is crucial. In this paper, based on the phase group equilibrium model, the two-phase flow and bubble aggregation in the dissolved air flotation device are simulated numerically and verified experimentally. The results show that the gas content in the contact zone increases with the increase of gas-liquid ratio and decreases with the increase of contact zone height, and the bubbles will agglomerate during the floating process, making the bubbles larger and less stable, and accelerating the floating speed, which will affect the gas content distribution in the contact zone. The microscopic simulation shows that large-sized microbubbles are more likely to be aggregated during the floating process, while small-sized microbubbles are more stable and less likely to be deformed and aggregated due to surface tension. The results of the study provide a reference for the selection of dissolved air flotation gas-liquid ratio and the study of gas content and bubble distribution in flotation process.