Abstract: Flotation is an effective method for reducing ash and improving quality in coal gasification fine slag, yet traditional alkane−based reagents perform poorly in this separation. Through flotation experiments and analytical testing techniques, this study investigated the performance and synergistic mechanisms of combined reagents using dodecylamine, dodecanoic acid, and dodecanol as auxiliary agents to dodecane. The results show that the combination of dodecane with dodecanoic acid or dodecanol enhances collectability without significantly reducing selectivity, whereas the combination of dodecane with dodecylamine improves collectability but markedly reduces selectivity. The results demonstrated that the combination of dodecane with dodecanoic acid or dodecanol respectively enhanced the collecting capacity with no significant reduction in selectivity, whereas the compounding of dodecane and dodecylamine led to an improved collecting capacity yet a remarkable decrease in selectivity. Based on these findings, the mixed reagent of dodecanoic acid and dodecane was identified as the optimal combination for the separation of fine coal gasification slag. The optimal dosage of this mixed reagent was determined to be 22 kg per ton of slag, with the optimal volume ratio of dodecanoic acid to dodecane at 2∶3. Under such optimal conditions, the dosage of the collector was reduced by 45% compared with the sole use of dodecane (40 kg/t), the recovery rate of combustibles reached over 90%, and the ash content of the concentrate was maintained at a relatively low level (approximately 30%). Mechanistic analysis reveals that all three auxiliary reagents can form hydrogen bonds with oxygen−containing sites on the surface of the slag, enhancing the adsorption capacity of the collector. However, dodecylamine exhibits strong frothing ability and forms electrostatic adsorption with inorganic components, leading to non−selective flotation of inorganic matter and reduced selectivity. In contrast, the polar group of dodecanoic acid contains more oxygen, which strengthens hydrogen bonding, while the electrostatic repulsion with inorganic components improves the selectivity of the collector. Dodecanol only forms hydrogen bonds and cannot enhance selectivity through electrostatic interactions, making the dodecanoic acid–dodecane combination the most effective. Additionally, the increased surface activity of the combined reagents provides thermodynamic conditions for better dispersion of the collector in the solution and optimized froth properties, which is another important reason for the enhanced performance. This study provides a reference for the research and application of combined reagents in coal gasification fine slag processing.