Here we report a universal and cost-effective strategy via a one-step levodopa (L-DOPA)/3-amino-propyltriethoxysilane (APTES) reaction modified hydrophobic membrane for highly efficient separation of oil-water emulsions. Interestingly, the newly designed composite membrane possesses both high hydrophilicity and underwater super oleophobicity. Under gravity-driven separation conditions, the membrane displays superior efficiency to separate oil-water emulsions with an ultrahigh water flux. The low cost, the simplicity of fabrication, and versatility of the L-DOPA/APTES coating may open up new insights to molecularly design commercial membranes with superior hydrophilicity and underwater super oleophobicity for oily water separation and purification.