Replacing the flammable liquid electrolytes with solid ones has been considered to be the most effective way to improve the safety of the lithium batteries. However, the solid electrolytes often suffer from low ionic conductivity and poor rate capability due to their relatively stable molecular/atomic architectures. In this study, we report a composite solid electrolyte, in which polyethylene oxide (PEO) is the matrix and Li 6.4 La 3 Zr 1.45 Ta 0.5 Mo 0.05 O 12 (LLZTMO) and Li 6.4 La 3 Zr 1.4 Ta 0.6 O 12 (LLZTO) are the fillers. Ta/Mo co-doping can further promote the ion transport capacity in the electrolyte. The synthesized composite electrolytes exhibit high thermal stability (up to 413 °C) and good ionic conductivity (LLZTMO-PEO 2.00 × 10 -4 S·cm -1 , LLZTO-PEO 1.53 × 10 -4 S·cm -1 ) at 35 °C. Compared with a pure PEO electrolyte, whose ionic conductivity is in the range of 10 -7 ~10 -6 S·cm -1 , the ionic conductivity of composite solid electrolytes is greatly improved. The full cell assembled with LiFePO 4 as the positive electrode exhibits excellent rate performance and good cycling stability, indicating that prepared solid electrolytes have great potential applications in lithium batteries.