Abstract: 3mm multi layer waveguide slot array antennas excel the single layer ones in the aperture efficiency and im pedance band, which have been employed in miniaturization high resolution radar detection. Because the wavelength is short at 3mm band, the machining errors influence the antenna performance. The paper analyses the machining process influences to the antenna parameters choices. The design method is optimized to satisfy the criterions of manufacturability. Through pro cessing a prototype of 3mm planar slot array, analyzing the error of slot parameters quantitatively, and comparing the simula tion and measured results of electrical performance, the validity and practicability of the design method was proved.