Ammonia is a major metabolic product of aquatic organisms. It is highly toxic to aquatic organisms, and therefore must be removed from aquatic ecosystem. A new photocatalytic system was designed to remove total ammonia nitrogen in water in this study. Titanium dioxide was suspended by disc air diffuser stone and activated by PL type UV-A lamp light to produce free electron and electron hole. The photocatalytic oxidation of total ammonia nitrogen was measured in different water (deionized water, tap water and seawater) that was aerated with different gases (air, oxygen and ozone). In deionized water, the photocatalytic oxidation rate of total ammonia nitrogen aerating with different gases were 0.04 ± 0.02, 0.07 ± 0.01, and 0.24 ± 0.02 TAN mg/hr, respectively. In tap water, the photocatalytic oxidation rate of total ammonia nitrogen aeration with different gases were 0.59 ± 0.14, 0.67 ± 0.15, and 0.71 ± 0.18 TAN mg/hr, respectively. In seawater, the photocatalytic oxidation rate of total ammonia nitrogen aerating with different gases were 1.22 ± 0.26, 1.45 ± 0.18, and 1.81 ± 0.34 TAN mg/hr, respectively. It indicated that the photocatalytic oxidation rate of total ammonia nitrogen was affected not only by ion in water, but also by the gases types of aerating. Ozone aeration was the best to enhance the photocatalytic oxidation rate of ammonia by titanium dioxide. Oxygen aeration was the follower and air aeration was the last.