Gel polymer electrolyte for flexible energy storage devices such as supercapacitors and rechargeable batteries attracts widespread attentions. Herein, we fabricate a flexible, transparent, and eco-friendly gel polymer electrolyte film based on biodegradable polymer of the carboxylated chitosan via phase separation of carboxylated chitosan hydrogel in hydrochloric acid. HCl is the key chemical for crosslinking of film forming, and the process is simple, rapid and non-polluting. The obtained carboxylated chitosan hydrogel film with excellent flexibility exhibits a highest electrolyte uptake rate of 742.0 wt%, and a highest ionic conductivity of 8.69 x 10(-2) S cm(-1). A symmetric all-solid-state supercapacitors using carbon cloth as current collectors, activated carbon film as electrodes, and carboxylated chitosan hydrogel film as a gel polymer electrolyte shows a high specific capacitance of 45.9 F g(-1) at 0.5 A g(-1), and the maximum energy density of 5.2 Wh kg(-1) at a power density of 226.6 W kg(-1). To our knowledge, this is the first time that carboxylated chitosan hydrogel film being used as gel polymer electrolyte for supercapacitors electrolyte. The good results indicate that the carboxylated chitosan as a new gel polymer electrolyte material has great potential in practical applications of all-solid-state, flexible, and portable energy storage devices.