The present invention discloses a flexible lithium ion battery including an outer packing and an electric core contained in the outer packing, the electric core includes an electrode pole piece and an electrolyte, wherein the electrode pole piece includes an isolation layer and positive electrode and negative electrode materials which are distributed on the isolation layer by cold compression, the isolation layer is one or more of a glass-fiber-modified polyethylene terephthalate insulation film, a glass-fiber-modified aromatic-ring polycarbonate insulating film, a fluorinated polyolefin insulation film and a glass-fiber-modified polyacrylate insulating film, the pressure used for the cold compression is 50-150T, the compaction density of the positive electrode material is 3.5-4.1g / cm<3>, and the compaction density of the negative electrode material is 1.5-1.7g / cm<3>. In addition, the present invention also discloses a method for preparing the flexible lithium ion battery.