High-entropy double perovskite SmBa(Mn0.2Fe0.2Co0.2Ni0.2Cu0.2)2O5+δ (HE-SBC) as a cathode material was prepared by a modified Pechini method, and the performance of HE-SBC with 10% (in mole fraction) Gd2O3-doped CeO2 (GDC) (HE-SBC-GDC) was optimized. The results show that the thermal expansion of Co ions caused by the change of valence state can be reduced due to the formation of high-entropy at B-site, thereby reducing the thermal expansion coefficient of SBC. The polarization impedance (Rp) of the HE-SBC symmetrical cell with yttria-stabilized zirconia (YSZ) as an electrolyte is 1.04 Ω·cm2 at 800 ℃ and the maximum power density and Rp of the anode-supported single cell are 683.53 mW/cm2 and 0.46 Ω·cm2, respectively. Furthermore, the catalytic activity of HE-SBC is improved by the addition of GDC［m(HE-SBC):m(GDC)=7:3］ due to the enlarged three-phase interface. The polarization resistance of HE-SBC-GDC composite cathode symmetric cell is only 0.09 Ω·cm2 at 800 ℃ and the maximum power density and Rp of the anode-supported single cell are 838.66 mW/cm2 and 0.12 Ω·cm2, respectively.