The invention relates to a composite 
solid electrolyte with high 
ionic conductivity and a preparation method thereof, belonging to the technical field of 
solid electrolyte for 
lithium ion batteries. The composite 
solid electrolyte is composed of inorganic solid electrolyte, 
polymer electrolyte and 
lithium salt. The 
mass ratio of inorganic solid electrolyte, 
polymer electrolyte and 
lithium salt is0.2-0.8: 0.2-0.8: 0.05-0.5. The invention prepares inorganic solid electrolyte nanofibers by electrostatic 
spinning, prepares vertically oriented inorganic solid electrolyte skeleton by freeze castingmethod, and pours 
polymer and lithium salt to form composite solid electrolyte. Nanofibers instead of nanoparticles have obvious advantages in the preparation of intact and uniform inorganic solid electrolyte skeleton and the formation of fast Li + transport pathway, which can significantly improve the 
room temperature ionic conductivity of the electrolyte, up to 10<-4>S.cm<-1>. By adjusting thesize of nanofibers and freeze-
casting conditions to control the skeleton structure, so as to regulate the comprehensive properties of the electrolyte.