The variation of pH at the interface in the static liquid-liquid interfacial polymerization was studied. And a positively charged composite nanofiltration (NF) membrane was fabricated by interfacial polymerization to achieve crosslinking of poly[2-(N,N-dimethyl amino)ethyl methacrylate]（PDMAEMA）and setting the PDMAEMA layer on polysulfone(PSF) membrane, using PDMAEMA as a pre-polymer, di-benzylic agent as a crosslinker and porous polysulfone (PSF) membrane as a substrate. The effect of pH on swelling ratio (SR) of the PDMAEMA hydrogel film and the separation performance of the PDMAEMA/PSF NF membrane were investigated. The results show that, the quaternized amino groups produced from interfacial polymerization are soluble in both phases, which accelerate the crosslinking reaction as self-catalysts; the interfacial crosslinking of a polyelectrolyte monomer on a substrate membrane should be performed in a specific range such as pH 7~9 for this reaction; the swelling ratio of pure PDMAEMA gel film exhibits obvious pH responsibility, and this property can be seen in the prepared composite NF membrane with PDMAEMA as top layer materials, which shows rejection and flux pH-responsibilities in NF process too.