A new method for fabrication of thin film nano fibrous composite (TFNC) ultrafiltration (UF) membrane consisting of an ultrathin poly(acrylonitrile-co-acrylic acid) (PAN-AA) barrier layer based on a polyacrylonitrile (PAN) nanofibrous support layer was proposed in this study. First, a thin PAN-AA nanofibrous layer was electrospun and deposited on a thicker PAN nanofibrous substrate. Then, the as-prepared PAN-AA nanofibers were swollen in the alkaline buffer solution and merged imperceptibly as an integrated nonporous hydrogel layer on the PAN substrate. The PAN-AA hydrogel layer was cross-linked with different bivalent metal cations(Ca2(+), Mg+) to form an ultrathin barrier layer, of which the thickness and porosity were optimized by controlling the depositing time of PAN-AA nanofibers and pH value of buffer solution. Proteins with different molecular weights were used to evaluate the ultrafiltration performance of the, resultant charged barrier layer, the PAN-AA-Mg and PAN-AA-Ca TFNC UF (221.2 and 219.2 L/m(2)h) and rejection efficiency (97.8% and 95.6%) at 0.3 MPa. The PAN-AA TFNC UF membranes could be used to composite membranes. Due to its hydrophilic and negative composite membranes exhibited excellent permeate flux for bovine serum albumin (BSA) aqueous solution (1 g/L) retain solutes, of which the radius was larger than 4.6 nm.