A high-aspect-ratio copper microchannel cooling device for radar was fabricated on metallic substrate based on the electrochemical deposition technique. In view of the problem of non-uniform thickness in making thick film with high-viscosity SU–8 photoresist, lapping and polishing techniques were applied to the unexposed SU–8 film to improve the thickness uniformity and the dimension error caused by air gap was also reduced. A novel method to measure the thickness of unexposed SU–8 film using refractive index was proposed and the refractive index of SU–8 photoresist at yellow sodium light was obtained by least square method. In order to overcome the failure in making high-aspect-ratio SU–8 film due to inappropriate exposure dose, the effect of exposure dose on the quality of the film was discussed by lithography experiment and the optimal parameter was 640mJ/cm2. SU–8 film with good quality was acquired and a copper microchannel cooling device was fabricated based on the above techniques and experiment. The width is 50μm, the height is over 250μm and the aspect ratio is over 5.