Abstract: Viscosity is an important factor affecting the high-value application of kaolin. In order to figure out how unavoidable ions influence the viscosity of kaolin, the slurry was investigated through a viscomete runder different concentrations of Ca²⁺ and Mg²⁺ ions. The interaction potential energy between kaolinite particles was calculated and analyzed based on the zeta potential and DLVO theory. The results showed that Ca²⁺ and Mg²⁺ increased the viscosity of kaolin slurry, with the enhancement due to Ca²⁺ being stronger than that of Mg²⁺. In addition, based on the zeta potential measurements, Ca²⁺ and Mg²⁺ increased the surface potential of kaolin slurry, but reduced the repulsive force between kaolinite particles, thereby promoting the flocculation of kaolinite particles and improving the viscosity of slurry. Moreover, DLVO theoretical calculation showed that an increase of Ca²⁺、Mg²⁺ concentration reduced the potential interaction barriers between the basal faces, as well as between the edge faces and basal faces of kaolinite particles.