The ambient temperature application of solid-state batteries (SSBs) based on ceramic/PEO composite solid electrolyte has been restrained by the poor ionic conductivity of PEO-based polymer. Herein, we have studied high-salt-concentrated polymeric electrolyte comprising poly(propylene carbonate) (PPC) and lithium bis-(fluorosulfonyl) imide (LiFSI), as well as its application in ceramic/polymer composite solid electrolytes based SSBs. The PPC-LiFSI 80 wt.% electrolyte showed a significantly improved ionic conductivity on the order of 10−4 S cm−1 at ambient temperature, Li transference number of 0.75, and anodic stability up to 4.5 V versus Li/Li+. The LiFePO4/Li cell with Li1.5Al0.5Ge1.5(PO4)3 (LAGP)/PPC-LiFSI 80 wt.% composite electrolyte showed stable charge/discharge profiles and excellent cycling performance, which delivered a specific discharge capacity of 138.3 mAh g−1 at 0.1C and a high capacity retention of 97.1% after 100 cycles. According to EIS results, we found that the reduced cell deterioration and polarization is attributed to the improved interface between the electrode and the solid electrolyte, which is realized by introducing the flexible and conductive high-salt-concentrated polymeric electrolyte PPC-LiFSI 80 wt.%. The results indicate that ceramic/high-salt-concentrated PPC-based polymer composite electrolyte is promising for ambient temperature solid-state lithium batteries.