The addition of electrolytes considerably increases the recovery of mineral matter in coal flotation. In this paper, the effect of calcium ion on coal flotation in the presence of kaolinite clay was investigated through microflotation experiments on a mixed coal kaolinite system. The interaction behavior and mechanism between coal particles and kaolinite were indicated by a rotational viscometer, scanning electron microscope (SEM), settlement tests, and atomic force microscope (AFM). The microflotation results reveal that the combustible recovery increased dramatically as the concentration of calcium increased, and then reached a maximum of 85.36% at 5 mmol/L calcium concentration, after which the recovery declined. The ash content of the concentrate always increased with increasing calcium concentration. The rheology measurements and SEM analysis show that the heterocoagulation between coal and kaolinite does not occur when the Ca2+ concentration is lower than 5 mmol/L. A kaolinite coating on the coal surface is confirmed in the Ca2+ concentration range of 5-8 mmol/L, leading to a sharp increase in the concentrate ash content. The settlement tests and AFM analysis show that the electrostatic repulsive force is fully suppressed by excessive Ca2+ addition and that the attractive force dominates the kaolinite particle interaction. These results can give a more detailed description of the flotation behavior of kaolinite in inorganic electrolyte solution and provide guidelines for the industrial application of saline water coal flotation.