A high-temperature proton exchange membrane is a key component of high-temperature proton exchange fuel cells. However, its development has been impeded by problems such as mechanical stability issues and free phosphoric acid leaching. Here, phosphonated phenol-formaldehyde is first introduced into polybenzimidazole as a high-temperature proton exchange membrane, which synchronously has sufficient intrinsic protonic conductors and long-range proton transport channels. We propose replacing free phosphoric acid with immobilized phosphoric acid based on phosphonated phenol-formaldehyde, thus balancing the relationship between electrochemical performance and mechanical stability in a phosphoric-acid-doped membrane. Phosphonated phenol-formaldehyde with immobilized phosphoric acid can not only achieve anhydrous proton conductivity at high temperatures, but it can also mitigate the adverse impact of phosphoric acid on the swelling of the membrane. A composite 50PPF/PBI membrane with a low acid uptake level reached a peak power density of 607 mW cm−2 at 160 °C under fully anhydrous conditions. A single cell based on 50PPF/PBI MEA has a low attenuation rate over a period of 50 hours due to the eminent PA retention properties of the membrane. This result indicates a pathway to using phosphonated polymers as immobilized phosphoric acids in the field of high-temperature proton exchange membranes.