A flexible battery is disclosed in the present invention. One of a first active material layer, an intermediate layer, a second active material layer, a first interface located between the first active material layer and the intermediate layer, and a second interface located between the second active material layer and the intermediate layer includes a first adhesive. The first adhesive includes at least one first linear polymer and at least one first crystallization inhibitor. Therefore, the active material layers, the intermediate layer or the interfaces have sufficient adhesion and flexibility. The electrochemical reaction element would not easy to be cracked or separated after bending, and the ionic and electrical conductivities of the battery is greatly improved.