According to the present disclosure, a plurality of aluminum oxide nanoparticles and a polymerizable monomer are dissolved in a polyimide solution to obtain a polyimide mixed solution. Next, the polyimide mixed solution is coated onto a glass substrate. Then, the surface of the glass substrate is irradiated with UV light to form a network polymer on the surface of the glass substrate, the network polymer enclosing the aluminum oxide nanoparticles. Finally, the glass substrate is heated to form the flexible material having the surface protecting layer. Abrasion resistance of the flexible material manufactured according to the present disclosure is excellent.