Selective catalytic reduction (SCR) technology is currently one of the most effective methods to reduce NOx emissions for engine. NH3-SCR technology is also considered to be the most promising hydrogen engine after-treatment device. This paper used Cu-SSZ-13, which is widely commercially available, as the research object, and explored the relationship between micron and nanoscale grain sizes through experimental methods such as BET, XRD, NH3-TPD, UV-vis-DRS and activity testing, the influence mechanism of micron-scale and nano-scale grain size on the morphology and properties of Cu/SSZ-13 catalyst was explored. The results show that the fresh nanoscale 900F sample has higher low-temperature NOx conversion efficiency, while the micron-scale 1800F sample has poor low-temperature activity and better high-temperature activity. This is closely related to its morphological characteristics, adsorption and desorption characteristics and dual-site properties. The specific surface area and total pore volume of the 900F sample are larger, but according to the diffraction peaks in XRD, its crystallinity is low, resulting in the high temperature activity of the 1800F sample being higher than that of the 900F sample. After SO2 poisoning, the proportions of strong Lewis acid sites and Brønsted acid sites increased, resulting in an increase in both low-temperature and high-temperature activities of the 900S sample. And the activity of micron-scale samples decreases sharply, and the high-activity temperature window shrinks.