Solid polymer electrolyte is strongly considered to be one of the key materials to solve safety problem of high energy lithium batteries caused by flammable liquid organic carbonate solvent, while the development and application of solid polymer electrolytes are seriously restricted by the low ambient temperature ionic conductivity. Recently, aliphatic polycarbonate-based electrolytes have been reported to possess high ionic conductivity at ambient temperature. Herein we choose poly (propylene carbonate) as a model to investigate the origin of the high ionic conductivity of polycarbonate-based polymer electrolytes. Our results prove that poly (propylene carbonate) degrades to micromolecular segments when contacts with Li-metal anode. The depolymerization products can infiltrate the interface and swell the electrolyte to develop the amorphous state so as to reduce both interfacial and bulk resistance. As scaffold of the electrolyte, cellulose can limit this reaction at the interface by physical obstruction. The hydroxyl units can also quench the lithium alkoxide intermediate to suppress further degradation, guaranteeing the security of the system. This study provides a profound understanding of the composition, reaction and evolution of the interface between polymer electrolyte and Li-metal anode, which sheds new light on the interface construction of ambient-temperature solid-state lithium metal batteries.