Electroforming of submillimeter scale array structures holds great promise for a wide range of applications in array microelectrodes, vacuum electron devices and metal molds, but poor and uneven mass transfer inside the submillimeter scale mask negatively affects the electroforming quality. To address this issue, a jet-flush mixed flow field is proposed here to fabricate a submillimeter array structure with an aspect ratio of more than 3. In this method, the jetting flow greatly reduces the effective diffusion layer thickness of the electrolyte, and the flushing flow improves the mass transfer uniformity inside the submillimeter scale mask. The simulation results indicate that the average effective diffusion layer thickness can be reduced by ~74 % and the standard error is controlled within ~7 μm by optimizing the jetting inlet angle, jetting velocity, and flushing velocity. The submillimeter scale array structures were fabricated by electroforming under a jet-flush mixed flow field (jetting inlet angle of 60°, jetting velocity of 3.2 m/s, flushing velocity of 0.4 m/s). This paper guides the high-quality electroforming of the submillimeter scale metal array structures with a high aspect ratio.