A series of poisoned catalysts with various forms and contents of sodium salts (Na₂SO₄ and Na₂S₂O₇) were prepared using the wet impregnation method. The influence of sodium salts poisoned catalysts on SO₂ oxidation and NO reduction was investigated. The chemical and physical features of the catalysts were characterized via NH₃-temperature programmed desorption (NH₃-TPD), H₂-temperature programmed reduction (H₂-TPR), X-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller (BET), X-ray diffraction (XRD), and Fourier Transform Infrared Spectroscopy (FT-IR). The results showed that sodium salts poisoned catalysts led to a decrease in the denitration efficiency. The 3.6% Na₂SO₄ poisoned catalyst was the most severely deactivated with denitration efficiency of only 50.97% at 350 °C. The introduction of SO₄²⁻ and S₂O₇²⁻ created new Brønsted acid sites, which facilitated the adsorption of NH₃ and NO reduction. The sodium salts poisoned catalysts significantly increased the conversion of SO₂–SO₃. 3.6%Na₂S₂O₇ poisoned catalyst had the strongest effect on SO₂ oxidation and the catalyst achieved a maximum SO₂–SO₃-conversion of 1.44% at 410 °C. Characterization results showed sodium salts poisoned catalysts consumed the active ingredient and lowered the V⁴⁺/V⁵⁺ ratio, which suppressed catalytic performance. However, they increased the content of chemically adsorbed oxygen and the strength of V⁵⁺=O bonds, which promoted SO₂ oxidation.