Hydrogel-based electrolytes are becoming generally more preferred for electrolyte materials for quasi-solid-state/flexible supercapacitors due to their shape-controlled, high ionic conductivity, no liquid leakage, certain mechanical strength, and flexibility. Here, we develop a modified supramolecular carboxylated chitosan hydrogel electrolyte (CYCTS-g-PAM-Li2SO4) synthesized through a free radical graft copolymerization of acrylamide monomers and chemical crosslinking. Compared with PAM-based hydrogel electrolyte (PAM-Li2SO4), the modified hydrogel electrolyte of CYCTS-g-PAM-Li2SO4 exhibits better performance including higher ionic conductivity (1.74 × 10−2 S cm−1), better electrolyte absorbability (280%) as well as improved mechanical properties (2.25 MPa). A symmetric quasi-solid-state EDLC (electronic double layer capacitor) by sandwiching activated carbon film electrodes with CYCTS-g-PAM-Li2SO4 as the electrolyte and carbon cloths as the current collectors. Notably, the device shows a wide potential window of 0–1.4 V and delivers a high energy density of 8.7 Wh kg−1 at a power density of 350.3 W kg−1. Interestingly, two devices connected in series can light an LED lamp. To our knowledge, this is the first time that reported the modified carboxylated chitosan hydrogel electrolyte for quasi-solid-state supercapacitors, which has application potentials for the modified carboxylated chitosan as hydrogel electrolyte for quasi-solid-state, flexible, and portable energy storage devices.