Five Cu-SAPO-18 (a silicoaluminophosphate molecular sieve) catalysts with different Si contents prepared by a one-pot method were used to investigate the effect of their Si content on the ammonia selective catalytic reduction (NH3-SCR) performance. Although the as-prepared catalysts exhibited similar Cu loading, they showed quite different NO conversions from 150 degrees C to 575 degrees C before and after hydrothermal aging treatment at 800 degrees C for 12 h. The XRD, Si-29 MAS NMR, NH3-TPD, H-2-TPR, EPR and kinetic tests were applied to characterize the various Si coordination structures in Cu-SAPO-18 samples and their roles on the NH3-SCR performances. The Si-29 MAS NMR, NH3-TPD and EPR results indicated that the varying proportion of Si could affect the Si coordination structures, and further affect the acid properties and Cu distributions in Cu-SAPO-18 catalysts. The amount of acid sites and isolated Cu2+ ions firstly increases and then decreases with increasing Si content. The NO conversion results indicated that catalytic activities are jointly decided by acidity and amount of isolated Cu2+ ions. On the other hand, the more Si (xOAl) (x=1-3) structures, the more strong acid sites could be generated in Cu-SAPO-18 catalysts. Furthermore, these strong acid sites are beneficial for the stabilization of isolated Cu2+ ions, and thus the stabilities of isolated Cu2+ ions and zeolite framework are enhanced during the harsh hydrothermal aging treatment. Additionally, the kinetic result suggested that Si content does not influence the apparent activation energy (E-a) of NH3-SCR over Cu-SAPO-18 catalyst at low temperatures.