Effects of Ca²⁺, Mg²⁺, Fe³⁺, Cl^-and SO₄^2-with different concent rations on bacteria growth and bactericidal perfo rmance when Niukouyu Reservoir wastewater is reused as cooling water have been studied.The results show that Ca²⁺can facilitate iron bacteria growth, but has no ef fect on bacteria.Mg²⁺ facilitates bacteria and iron bacteria growth.More Fe³⁺goes against iron bacteria growth, but has no effect on bacteria grow th.All three ions have no effect on the bactericidal perfo rmance of dichloro-iso-cyanuric acid sodium salt and dodecyldimethyl benzy lammonium chloride.Cl^-concent ration between 100 ～ 400 mg/L has no effect on the bactericidal performance of dichlo ro-iso-cyanuric acid sodium salt.The same result is acquired while SO₄^2- concentration is between 100 ～ 300 mg/L.Iso thiazolin is not suitable for being used as bactericide in Niukouyu wastewater reuse.