La0.9Sr0.1Ga0.8Mg0.2O2.85(LSGM) powders were synthesized by solid state reaction, glycine-nitrate process and acrylamide polymerization, respectively. Novel composite electrolytes were obtained by mixing LSGM powders with binary carbonates. The conductivity behavior of LSGM-carbonate composite electrolytes has a transition temperature. The conductivities of the composites are much higher than that of pure LSGM, reaching 0.122Scm-1 at 600℃ and 0.08Scm-1 at 450℃，The maximum power densities reach 617mWcm-2 at 600℃ and 311mWcm-2 at 450℃ for SOFC with LSGM-carbonate composite electrolytes. The output performance and OCV is influenced by LSGM morphology,size and carbonate content. and the composite electrolyte with 20 wt % carbonate show the best performance.