Abstract: P25 TiO₂ is applied to the photocatalytic degradation of ammonia-nitrogen wastewater to treat ammonia-nitrogen polluted water. The effects P25 dosage, pH value of reaction solution, initial concentration of ammonia-nitrogen, additional oxidant of H₂O₂ and common ions (Cl^-, HCO₃^-, NO₃^-, SO₄^2-, Na⁺, K⁺, Ca²⁺, Mg²⁺) in the water on removing ammonia-nitrogen are discussed. The optimum factors to ammonia-nitrogen (100 mg/L) degradation under the irradiation of 500 W lamp are 0.5 g/L of P25, initial pH 10.1 and 2 mmol/L of H₂O₂, and the removal efficiency is up to 59.4 %. In addition, ions of Cl^-, HCO₃^-, NO₃^-, SO₄^2-, Na⁺, K⁺, Ca²⁺ and Mg²⁺ all inhibit the degradation of ammonia-nitrogen. This study indicates that P25 has the photocatalytic ability to degrade ammonia-nitrogen in water.