Coal gasification fine slag (CGFS) produced by a Texaco coal water slurry gasifier in Shaanxi, China was taken as the research object. The carbon-ash separation of CGFS was carried out by flotation with composite collector. The raw material and the flotation product were characterized by XPS, BET, SEM–EDS, et al. analysis. The results showed that when the dosage of composite collector was 15 kg/t and MIBC was 4 kg/t, the flotation perfect index and the combustible recovery were 47.78 and 84.34%, respectively, which increased by 14.69 and 23.03% compared with dodecane as collector, but the concentrate ash content was not significantly reduced. XPS analysis shows that there are still a large number of hydrophilic groups on the surface of the unburned carbon in the tailings, and these hydrophilic groups hinder the mineralization process of the unburned carbon and collector, thus affecting the floating of residual carbon. BET and SEM analysis results showed that the specific surface area of the flotation concentrate was 303.50 m2/g and the pore structure was developed, and EDS analysis showed that a large number of fine-grained spherical inorganic matter was still filled in the pore structure of unburned carbon, which was the main reason why the concentrate ash content was still about 30%.