Abstract: In the laboratory and with a simulated wastewater as samples, the factors influencing the precipitation of ammonia nitrogen from wastewater were investigated with MgCl₂ and Na₂HPO₄ as precipitants. These factors studied include the pH of precipitation, the dosage of precipitants, and the initial NH₄⁺-N concentration of waste-water. The result shows that the pH of precipitation is the key factor and it determines the precipitation rate of NH₄⁺-N, the remaining concentration, the removal rate and the remaining concentration of Mg²⁺ and PO₄^3-. The optimum pH is 11. When the initial NH₄⁺-N concentration is less than 1 000 mg/L, both the precipitation rate and the residual concentration of NH₄⁺-N decrease with decreasing the initial concentration. When the initial NH₄⁺-N concentration is less than 100 mg/L, the precipitation rate decreases dramatically, but the minimum residual concentration is 5 mg/L and remaining the same.