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Design and fabrication of phosphoric acid (PA)-doped polybenzimidazole (PBI) membranes with high proton conductivity, mechanical property and phosphoric acid retention stability has been a critical challenge. In this study, a sulfonated ionic covalent organic framework (SCOF) was in-situ synthesized in the polybenzimidazole matrix. The high-density sulfonic groups and ordered channel structure of SCOF rendered the as-prepared composite membrane high proton conductivity under low phosphoric acid doping level. Additionally, the introduction of SCOF helped to immobilize the phosphoric acid, thus effectively reducing its loss and improving the acid retention stability. The acid-base interaction formed between the sulfonic acid and imidazole groups act as ionic crosslinkers, which significantly enhanced the mechanical-dimensional stability of composite membranes. As a result, the PA-PBI/SCOF composite membrane exhibited a high proton conductivity up to 542.1 mS cm−1 at 80 °C, 100 % RH and 134.1 mS cm−1 at 30 °C, 50 % RH, as well as a high mechanical strength up to 64.0 MPa (PA uptake: 67.47 %). This in-situ synthesis strategy offers a promising new approach to fabricate covalent organic framework-based composite membranes for various applications. [Display omitted] • A sulfonated covalent organic framework (SCOF) was in-situ synthesized in polybenzimidazole matrix to prepare composite membranes. • The as-prepared composite membrane exhibited high mechanical strength, proton conductivity and phosphoric acid retention stability. • The composite membrane showed comparable performance in electrochemical hydrogen compression and proton exchange membrane fuel cell. [ABSTRACT FROM AUTHOR]