Herein, poly[2,2′-(p-oxydiphenylene)-5,5′-benzimidazole] (PBI) is synthesized from 3,3′-diaminobenzidine and 4,4′-oxybisbenzoic acid, and the membrane is prepared by solvent casting. The main characteristics of PBI are studied. In the preparation of the PBI/H3PO4 composite membrane, the absorbing temperature of H3PO4 is 120 °C, which leads to a membrane with a high content of H3PO4. Membrane electrode assemblies (MEAs) are fabricated from PBI/H3PO4 membranes with the catalyst layer made of Pt/C, PBI, and polyvinylidene fluoride (230:12:7 w/w). The fabricated MEA is tested at 150 °C with dry hydrogen and oxygen gas at 0.2 MPa for both anode and cathode feeds. No degradation of voltage is seen during stability testing of the PBI/H3PO4 membrane at a constant current for 100 h. The maximum power density is 1.17 W cm−2, and the maximum current density is 6.0 A cm−2 with a Pt loading of 0.5 mg cm−2. The high performance of these membrane materials demonstrates that PBI can be regarded as an alternative membrane material for high-temperature proton-exchange-membrane fuel cells.