A surface-modified separation membrane of the present invention comprises a separation membrane, and a coating layer formed on the surface of the separation membrane for improving the contamination resistance and chemical resistance of the separation membrane, wherein the coating layer is implemented with a nanoscale thickness of the coating layer in order to inhibit a decrease in permeation flux of the separation membrane before and after coating the coating layer, and comprises: dopamine for providing, to the coating layer, an adsorption force to be bound stably with the separation membrane; and a hydrophilic material which is bound to the dopamine through secondary bonding or cross-linking containing a hydrogen bond in order to inhibit the deterioration of the durability of the coating layer, and provides hydrophilicity to the surface of the separation membrane in order to protect the separation membrane from hydrophobic contaminants. A method for modifying the surface of a separation membrane of the present invention comprises the steps of: injecting a separation membrane to be surface-modified into a reactor; adding a mixture solution, which is formed by adding a hydrophilic material to a Tris-buffer solution having an adjusted pH range at which dopamine reacts and stirring the same, to the reactor; adding dopamine and an initiator, which induces the cross-linking of the hydrophilic material and dopamine, to the reactor at a predetermined temperature range; and forming a coating layer comprising dopamine and the hydrophilic material on the separation membrane through thermal cross-linking at the predetermined temperature range while injecting oxygen into the reactor.