Argyrodite Li6PS5Cl is a promising solid electrolyte in all-solid-state lithium batteries owing to its high ionic conductivity. However, the poor mechanical property and undesirable interfacial property restrict its application. In order to solve these issues, we fabricate Li6PS5Cl/poly(ethylene oxide) composite solid electrolytes with enhanced mechanical property and stable lithium/electrolyte interface. By adding 5 to 20 wt% poly(ethylene oxide), the proportional limit of composite solid electrolytes is enhanced by ∼150%, reaching a value up to 60 MPa. With 5 wt% poly(ethylene oxide), the as-assembled sandwich-type LiNi0 · 8Co0 · 1Mn0 · 1O2/composite solid electrolytes/Li battery exhibits enhanced cycling performance with a capacity retention rate of 91% over 200 cycles at 0.05C and 30 °C. Ex situ characterizations reveal that by adding suitable poly(ethylene oxide) content in Li6PS5Cl, the interfacial reactions and lithium dendrite growth can be effectively inhibited, resulting in much improved cycling performance of all-solid-state lithium batteries.