Fabrication of novel composite proton exchange membranes (PEMs) with efficient proton transfer sites and homogeneous structure is crucial to PEM fuel cells but remains challenging. In this study, organic polydopamine sol is synthesized by a facile self-polymerization method and functionalized by abundant - PO3H2 groups, then the composite membrane is fabricated by mixing the functionalized polydopamine sol with Nafion followed by polycondensation of sol. The electrostatic interactions between functionalized polydopamine (FDPA) networks and Nafion matrix confer the composite membrane with excellent interfacial compatibility and enhanced physical, chemical stabilities. Due to the introduction of -PO3H2 groups, the hydrogen-bonded water bridges can be easily constructed and the energy penalty for proton transfer is greatly reduced. Meanwhile, the tradeoff effect between proton conductivity and fuel permeability is surmounted. The composite membrane displays the highest proton conductivity of 0.0314 S cm(-1) (80 degrees C, 40% RH) and selectivity of 7.94 x 10(4) S s cm(-3) (25 degrees C), which is 15.2 times and 4.5 times higher than those of the recast Nafion, respectively. Compared with recast Nafion, a 58.4% increase in maximum power density is achieved for the Nafion/FDPA-10 composite membrane.