Abstract: The solid-liquid separation of ultra-fine gasification slag in the cooling water of gasification flue gas from a petrochemical plant was explored by flotation. On the basis of conventional flotation, the effects of the addition of the adjusting agent polyethylene oxide (PEO) and hydrodynamic cavitation pretreatment on the flotation of this gasification slags were investigated, and the intrinsic mechanisms of PEO and hydrodynamic cavitation pretreatment affecting the flotation of the gasification slags were initially explored based on modern detection methods such as focused beam reflection (FBRM), particle video microscopy (PVM) and dynamic light scattering (DLS). The flotation test results show that the flotation recovery of gasification residue is increasing with the increase of diesel oil and 2-Octanol, but there is a problem of high chemical consumption under high recovery. Both PEO addition and hydrodynamic cavitation pretreatment can enhance gasification slag flotation. Cavitation pretreatment of tap water used for slurry preparation and PEO addition during flotation conditioning can achieve synergistic enhancement of gasification slag flotation by both treatments, but PEO addition before cavitation pretreatment weakly promotes flotation efficiency by both treatments. The results of the mechanism study showed that both PEO addition and hydrodynamic cavitation pretreatment of tap water could promote the agglomeration of particles in the pulping process and increase the apparent particle size of gasification slag particles. The hydrodynamic cavitation process produced a large number of micro and nano bubbles, which significantly reduced the bubble population size distribution of the flotation system. However, the addition of PEO during the hydrodynamic cavitation pretreatment process led to an increase in the average size of the obtained micro-nano bubbles, probably because the presence of PEO does not facilitate the generation of micro-nano bubbles at the microfine particle level, thus weakening the subsequent flotation enhancement.