The all-solid-state Li-air battery is a promising next-generation energy storage technique because of its high safety and high theoretical specific energy density. The all-solid-state Li-air batteries with LAGP as electrolyte layer, Li foil as anode and a mixture composed of single-walled carbon nanotubes (SWCNTs), RuO2 nanoparticles and Li1.5Al0.5Ge1.5P3O12 (LAGP) as composite cathode were investigated in ambient air at room temperature. Charge overpotential was reduced because of the superior catalytic activities of RuO2 nanoparticles. Besides, SEM images showed the formation of chip-like products after discharge to 1000 mA h g-1 and wrinkles-like products after fully discharged to 2 V, respectively. Galvanostatic intermittent titration technique (GITT) demonstrated that the equilibrium voltage of all-solid-state Li-air batteries cycling in oxygen kept stable at 2.96 V while that cycling in ambient air maintained at 3.15 V. These results indicated that the electrochemical reactions of all-solid-state Li-air batteries can be ascribed to the formation and decomposition of lithium peroxide in oxygen and electrochemical reaction with moisture in air.