Novel composite nanofiltration membranes (NFMs) were simply fabricated via co-deposition of mussel-inspired polydopamine (PDA) and polyetheylenimine (PEI) followed by glutaraldehyde (GA) crosslinking. A uniform, robust and defect-free selective layer was generated on the hydrolyzed polyacrylonitrile (HPAN) ultrafiltration membrane substrate, endowing the composite NFMs with high separation performance for multivalent ions. Zeta potential measurements indicate these NFMs are slightly positively charged, resulting in a salts rejection sequence of MgCl2>CaCl2>MgSO4>Na2SO4>NaCl at pH 5.5. The nanofiltration performance can be tuned by changing the co-deposition time and the mass ratio of dopamine/PEI. A mass ratio of 2/2 with 4 h co-deposition is the optimum protocol for the membrane performances including surface hydrophilicity, water flux and salt rejection. Moreover, the composite NFMs show a good structural stability for immersing in ethanol or for long-term nanofiltration process.