Laboratory and field experiments were carried out to study the effect of calcium ions on coal flotation. Tap water and recycled water were used as flotation water in the laboratory experiments. Gypsum and potassium sulfate were used as modifying agents to adjust the water chemistry of flotation water. Gypsum was also used as a modifying agent to adjust the Ca2+ concentration of flotation water in field experiments. Both tap water and recycled water were used as flotation water in laboratory flotation tests, of which the results indicated that cleans ash increased by 1-2% with the increase of Ca2+ concentration. No obvious effect was found on cleans yield. The cleans ash gradually increased with the increase of Ca2+ concentration which is less than 5 mmol/L. Further increasing the concentration of Ca2+ did not cause further increase in cleans ash, but there was a slight fluctuation because of experimental error. The field flotation results indicated that cleans ash increased by 1.1% with the increase of Ca2+ concentration, which was consistent with the results of laboratory flotation tests. Since calcium ions had a negative effect on coal flotation, the dosage of gypsum used to treat coal tailings water should be controlled optimally.