A series of innovative branching OPBI composite membranes (OPBI-BTZ-PIL) were created based on benzotriazole side chain grafted OPBI (OPBI-BTZ) and double-bond ionic liquids for vanadium redox flow battery applications. The benzotriazole contained three nitrogen sites, which helped enhance the proton conductivity and low area resistance. The cation in the polymeric ionic liquid in the matrix and the vanadium ion produces the Donnan effect, which reduces vanadium permeability. Moreover, the hydrogen bond cross-linking network formed by the polymeric ionic liquid in the membrane further helps proton transport. The vanadium permeability of OPBI-BTZ₁₀-PIL₁₀ is as low as 1.51 × 10⁻⁹ cm² min⁻¹, which is one 40th of the Nafion117 (62.54 × 10⁻⁹ cm² min⁻¹). Its ion selectivity is up to 2.45 × 10⁶ S min cm⁻³. The OPBI-BTZ₁₀-PIL₁₀ membrane performs well at 60 mA cm⁻², with CE of 94.12% and EE of 86.2%, both of which are higher than the Nafion117 membrane.