The carbonates composite electrolytes for solid oxide cells have aroused more attention. However, the thermal stability of carbonates composite electrolytes remains in doubt due to the molten state of the carbonates at operating temperature. The novel composite electrolytes were obtained by mixing LSGM powders which were synthesized by acrylamide polymerization with binary carbonates. Ageing, cycling and longtime fuel cell performance tests were used to evaluate the stability of LSGM-(Li/Na)2CO3 composite materials. The sample shows a relatively stable conductivity of 0.07-0.09 Scm-1 during a 650 h measurement in air at 600℃.The composites keep weight stable during thermal cycling below 700℃, and demonstrate a good stability after 5 thermal cycles. The cell based on LSGM-(Li/Na)2CO3 composite electrolyte delivers an average output of 256 mWcm-2 in 1h operation corresponding to the constant current density 0.6Acm-2.