In this paper, we successfully synthesize phosphoric acid functionalized graphene oxide (PGO) based on acid modification of graphene oxide. The composite membrane is further prepared by adding PGO into sulfonated poly­(aryl ether ketone sulfone) containing carboxyl groups matrix (C-SPAEKS). The PGO as well as the composite membranes were characterized by a series of tests. The prepared composite proton exchange membranes (PEMs) have good mechanical and electrochemical properties. Compared to the C-SPAEKS membrane, the best composite membrane has a tensile strength of 40.7 MPa while exhibiting superior proton conductivity (110.17 mS cm–1 at 80 °C). In addition, the open-circuit voltage and power density of C-SPAEKS@1% PGO are 0.918 V and 792.17 mW cm–2, respectively. Compared with C-SPAEKS (0.867 V and 166 mW cm–2), it can be seen that our work has a certain effect on the improvement of the single cell performance. The above results demonstrate that the functionalized graphene oxide has greatly improved the electrochemical performance and even the overall performance of PEMs.