The invention provides a flexible lithium ion battery. The flexible lithium ion battery comprises a flexible battery case and a main battery body arranged inside the flexible battery case, electrolyte is filled in the flexible battery case, and the main battery body comprises a positive electrode plate, a diaphragm and a negative electrode plate which are stacked in sequence. The negative electrode plate is made from one of zinc cobaltate/carbon cloth composite negative electrode material, zinc oxide, tin oxide, ferric oxide and nickel oxide, and the positive electrode plate is made from one of lithium cobaltate, lithium iron phosphate and lithium manganate. The invention also provides a method for preparing the battery. In the invention, the problems that lithium ion batteries in prior arts have poor flexibility, relatively low capacitance, unsatisfactory circularity and poor electric stability are solved. The packaging of the battery is relatively simple, efficient and quick, most raw materials are low in cost, and the battery is suitable for popularization and has high practicability.