The influence of post-synthesis treatment with MnSO4, (NH4)2SO4 and (NH4)2S2O8 blended solution on NH3-SCR activity and hydrothermal stability of Cu-SAPO-18 (Cu-18) was investigated. Via adjusting the mixed solution concentration, the optimal Mn/Cu-18-III with higher deNOx activity and hydrothermal stability was acquired. Based on the optimal Mn/Cu-18-III, diverse metal cations such as La3+, Ce3+, Sm3+ or Zr4+ were simultaneously doped into Cu-18 during Mn2+ and NH4+ blended solution treatment to further promote its catalytic performance before and after hydrothermal treatment. The doping with Sm could improve the NH3 activity and hydrothermal stability of Mn/Cu-18-III to the greatest extent. The elevation of acid sites content and the disappearance of CuO species in Mn/Cu-18-III caused the enhancement of its NH3-SCR activity. More Cu2+ and acid sites in SmMn/Cu18-III were responsible for its higher NH3-SCR activity than Mn/Cu-18-III. The decline of vulnerable Bronsted acid sites by substituting Mn2+ or Sm3+ for H+ could restrain the hydrolysis of Cu-18 and enhance the hydrothermal stability of Cu-18 during hydrothermal treatment. The mechanism analysis demonstrates that the fresh and aged Cu-18, Mn/Cu-18-III and SmMn/Cu-18-III follow both Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) mechanism in NH3-SCR reaction, with the L-H mechanism being more dominated.