We report a method for generating laminar flows in double-layer microfluidic chips which can be used as photomasks. UV light could be transmitted selectively through the alternating of the two contacting fluids with different UV transmittances, and line patterns could be generated by crosslinking a photopolymerizable material at the bottom of the microfluidic photomask. The feature scales such as width and interval of the line patterns could be controlled actively and precisely by adjusting the flow rates of two different fluids.