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An insufficient amount of NH3 (ammonia) will reduce the conversion efficiency of NOx, which may lead to excess NOx emissions, resulting in NH3-SCR failure. In this article, SCR failure caused by a low NH3- NOx ratio is studied systematically by experiments. The main reasons for a low NH3-NOx ratio in SCR include insufficient urea injection, hydrothermal aging of catalysts and urea crystallization. It was found from an insufficient urea injection experiment that with the increase of NH3- NOx ratio, the NOx conversion efficiency of the SCR system increased, but the ammonia leakage also increased. The main influencing factors of NOx conversion efficiency are different under different NH3- NOx ratios. A flow reactor system was used in the catalyst hydrothermal aging experiment to investigate the effect of hydrothermal aging on catalyst activity. After a 24 h hydrothermal aging experiment at 800 °C, the NOx conversion efficiency of the copper-based zeolite catalysts decreased significantly at the boundary of medium and low temperature regions. And the NO2- NOx ratio in the mixture had a significant effect on the catalytic performance. Thermogravimetry coupled to Fourier transform infrared spectroscopy (TG-FTIR) was used to analyze the composition of urea deposits in a urea deposits analysis experiment. It was found that the main components of urea deposits were urea and isocyanic acid (HNCO). Preventing HNCO polymerization, especially the formation of CYA, can decrease the formation of urea deposits.