Janus membranes (JMs) have attracted increasing attention in the fields of liquid manipulation owing to its interesting liquid-unidirectional transportation feature. Nonetheless, most of the currently available methods to fabricate JMs suffer from specific or complicated operations, impeding practical applications of JMs. Herein, a facile liquid/liquid (immiscible oil/water) interface-confined surface engineering strategy was first applied on hydrophilic cotton fabric membrane to construct JM with the aid of mussel-inspired chemistry. The water-infused fabric channels and the unique distribution of amphiphilic C-18-NH2 at the oil/water interface enable the unilateral hydrophobicity transformation on fabric. As a result, the as-prepared membranes exhibited asymmetric chemistry, wettability, and surface morphology. The evaluation of water droplet behaviors across the membrane at the oil/water interface verified the water unidirectional transport feature of our JM. In addition, two collectors made by our JMs successfully demonstrated the excellent ability of water collection from both oil/water mixture and water-in-oil emulsion. The newly developed liquid/liquid interface-confined surface engineering strategy provides enormous potential for Janus membrane construction to manipulate liquid transportation towards smart applications.