This study develops a measurement method for testing the efficiency of photocatalysts in the degradation of NH3 gaseous concentrations. The catalysts used in this study are the same as those used previously in our UV/VIS spectrophotometer. Reaction measurements were carried out at 27°C and 17°C under ambient pressure on Heat Ventilating and Air Conditioning (HVAC) system. Quartz plates were coated with TiO2 nanocatalyst. Then the plate was put in the stable reaction cell under different flow rates of circulation. The experiment gives a direct and simple measurement of ammonia flux and hence of overall ammonia emission rate. The results reveal that the Submerged Arc Nanoparticle Synthesis System (SANSS) TiO2 nanocatalyst has excellent degradation efficiency towards NH3, so that when it is exposed to UV irradiation for 60 minutes, the gaseous concentration can be reduced to 10% of the original concentration. In addition, the rate constant of the degradation reaction of the self-made TiO2 nanocatalyst towards NH3 is as high as 0.039min-1.