Objective To solve the problem of shrinkages of cathodic electrophoretic coating film on die-cast aluminum alloy assembly. Methods By analyzing the causes of shrinkages of cathodic electrophoretic coating film, parameters of tank liquor in skimming section, forming section and electrophoresis section were adjusted. The duration of skimming section was extended. After being coated under 180 V voltages, the assembly was roasted and solidified. Results Upon inspection, the film on the assembly was flat and smooth and the shrinkage was eliminated. Salt spray resistance could up to 1000 h. The product acceptance requirements of the overall performance of the film were satisfied. Conclusion By adjusting the pretreatment process and the construction parameters of electrophoresis coating, the shrinkages on the die-cast aluminum alloy assembly is eliminated and the antiseptic property of ammo is improved.