The invention provides a flexible composite 
solid-state 
electrolyte, a full-
solid-state 
lithium-
ion battery and a preparation method thereof. The 
solid-
phase mixing of a 
sulfide solid-state 
electrolyte or a modifier thereof, a 
thermoplastic polymer or a modifier thereof and 
lithium salt enables the composite solid-state 
electrolyte to have good flexibility while improving the dispersion uniformityand effective contact of each component. A 
halide, 
phosphate and / or an 
oxide are added to the 
sulfide material especially before the composition of the 
sulfide solid-state electrolyte and the polymersolid-state electrolyte, thereby providing a multi-dimensional channel for 
lithium ion transmission, increasing the disordered degree of lithium 
ion distribution, and being capable of further improving the lithium ion 
conductivity and electrochemical stability of the composite solid-state electrolyte. The flexible 
composite electrolyte has the advantages of high lithium 
conductivity at the 
room temperature, good electrochemical stability, easy preparation and 
processing, ability of being bent and 
cut and the like. The formed flexile full-solid-state battery has good mechanical performance andbending performance, and improves the cycle life and 
energy density.