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Wire electrochemical micromachining (WEMM) has increasingly come to be recognized as a flexible and effective method for producing planar contour microfeatures. The machining performance of WEMM is significantly influenced by the mass transport rate in the machining gap. Recently, it has been found that the use of a wire electrode with surface microstructures further increases the mass transport rate. Nevertheless, the mechanism through which the microstructures on the wire surface affect the flow field in the machining gap has been poorly understood. In this paper, a simplified mathematical model taking account of the velocity slippage caused by hydrogen bubbles attached to the axially traveling wire surface is first established. The effects on the flow field distribution in the machining gap of surface hydrophilicity and microstructures on the wire surface are theoretically analyzed. The results show that the microstructures facilitate rapid detachment of hydrogen bubbles from the wire surface, resulting in a reduction in velocity slippage and increasing the mass transport rate. A series of comparative experiments on WEMM using both smooth wires and microstructured wires prepared by laser direct scanning machining are conducted. The results are in accordance with the theoretical analysis.