Abstract: The solid-liquid separation in coal slurry is a bottleneck that restricts coal processing and utilization. To clarify the influence of metal cation on the settling performance of coal slurry, the effect of metal cation on settling velocity and supernatant turbidity is investigated by the settling tests. Then the Zeta potential, focused beam reflectometer （FBRM）, image analysis system, and quartz crystal microbalance （QCMD） analysis are conducted from the perspective of the flocs characteristics. The results indicate that as the valence state of metal cation increases, the settling velocity of coal slurry and the transmittance of the supernatant both increases. High valence metal cations can better compress the double layer on the surface of particles, reduce electrostatic repulsion, and promote the agglomeration of coal slurry particles. The reduction in the number of coal slurry particles is the largest after adding Al³⁺, followed by Fe³⁺, Mg²⁺, and Ca²⁺, while the reduction in the number of Na⁺ and K⁺ is the smallest. The aggregation effect of metal cation on fine particles is significant, which can improve the compactness of flocs. When high valence metal cations are combined with APAM, the size and density of coal slurry flocs increase, and the pores and small branches at the edges of the flocs decrease. When Al³⁺ is combined with APAM, the size of coal slurry flocs is 2.964 mm and the floc density is 1063.69 kg/m³. The QCMD results reveal that as the ion valence state increases, the thickness of the adsorption layer gradually increases. The adsorption thickness of Fe³⁺and Al³⁺on the coal surface is 9.50 nm and 9.70 nm, respectively. The higher the valence state of metal cation, the lower the elastic modulus adsorbed on the coal surface and the looser the adsorption layer. Therefore, APAM is more likely to pass through the adsorption layer and flocculate coal particles through hydrogen bonding. In addition, the looser the structure of the adsorption layer, it is easier to bind with APAM through electrostatic interactions. Moreover, their larger and denser flocs are more conducive to the flocculation and settling of coal slurry. This study can provide theoretical basis for the agent system optimization of coal slurry settling and dewatering.