SmBaCoFeO_5+δ (SBCFO) double perovskite material as a high-performance cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs) was synthesized by a citrate-nitrite combustion process. The crystal structure, thermal expansion coefficients, ionic-electronic transport property, electrochemical performance and cell power density of the material were evaluated. The results show that SBCFO has an orthorhombic structure Pmmm at room temperature, but transforms into a tetragonal structure P 4/mmm at 300 ℃. The thermal expansion coefficients of SBCFO are 15.7×10^-6 K^-1 and 19.6×10^-6 K^-1 at 25-200 ℃ and 200-900 ℃, respectively. The electrical conductivity of SBFCO is higher than 100 S/cm at 600-800 ℃. The synthesized SBCFO exhibits a fast oxygen surface exchange and bulk diffusion kinetics, thus having a low polarization resistance. The area specific resistances of the SBCFO/La_0.8Sr_0.2Ga_0.8Mg_0.2O₃(LSGM)/SBCFO symmetrical cell are 0.210, 0.100, 0.054, and 0.032 Ω·cm² at 700, 750, 800, and 850 ℃ in air (50 mL/min), respectively. The rate-determining step of the oxidation-reduction reaction is the dissociation of adsorbed molecular oxygen into atomic oxygen. The electrolyte supported single cell, assembled with Ni-Gd_0.1Ce_0.9O_2-δ as an anode, LSGM (300 μm) as an electrolyte and the SBFCO as a cathode, delivers a maximum power density of 831 mW/cm² at 800 ℃.