A NASICON-based mixed-potential gas sensor utilizing the perovskite-type orthoferrite LaxSm1-xFeO₃(x = 0.2, 0.4, 0.5, 0.6, 0.8) sensing electrode (SE) was designed for SO₂detection. The research mainly put emphasis on the effect of different La³⁺doping ratios in LaxSm1-xFeO₃on the sensing characteristics, and the highest response value (-86.5 mV) to 1 ppm SO₂was observed for the sensor using La0.5Sm0.5FeO₃-SE. The response value of the fabricated sensor displayed segmentally linear relationship with the logarithm of SO₂concentration in the ranges of 0.005-0.2 ppm and 0.2-5 ppm, and the slopes were -8 and -86 mV/decade, respectively. Otherwise, the low detection limit of the present sensor even could reach to 5 ppb with an accurate response value (-8.4 mV). Meanwhile, the sensor also showed good repeatability, long-term stability and selectivity toward other interfering gases. Furthermore, the electrochemical impendence curves indicated the reason that the sensor attached with La0.5Sm0.5FeO₃-SE displayed the higher response values than the other sensors. The result indicated that differences of electrocatalytic activity was caused by diverse La³⁺doping content in sensing electrode. The sensing mechanism related to the mixed potential was demonstrated by the polarization curve.