Balancing phosphoric acid (PA) uptake with the dimensional stability of PA-doped membranes is a major challenge in the field of high-temperature proton-exchange membranes. Crosslinking is a common method to solve this problem. However, the construction of conventional crosslinked membranes usually involves a two-step process in which linear polymers are first prepared, followed by the addition of crosslinkers during film formation to generate crosslinked networks. This further complicates the preparation process of crosslinked membranes. In this study, superacid-catalyzed Friedel–Crafts polyhydroxyalkylation was employed to fabricate knitted-network membranes with a high density of alkaline groups in a single step, significantly simplifying the preparation process of crosslinked membranes. Compared with commercial OPBI membrane, the knitted-network membranes exhibited significantly improved mechanical properties; moreover, PA retention was significantly improved, attributable to the formation of three-dimensional crosslinked networks and ultra-microporous structures within the membranes. Additionally, the PA-doped knitted-network membranes exhibited a good conductivity of 186 mS cm⁻¹ at 200 °C and a high peak power density of 850 mW cm⁻² at 200 °C without backpressure and humidification. This study is the first to report this in situ polymerization method in which the same precursor monomer is adopted in the subsequent polymerization process to form a crosslinked network. The proposed strategy exhibits significant application potential in the field of high-temperature proton-exchange membrane fuel cells.