The purpose of this research is to make design automation possible by parametric design and design associability in 3D CAD. Applications of the technology in liquid food packaging machinery and its advantages are presented in the paper. For regular shapes such as circles or lines, parameters are used to link the dimensions of relative parts while for irregular shapes such as curves, associability is used to do the job in an assembly. Commercial 3D CAD software and Microsoft Excel software were used for the application. It is proved that same merit applies to designs of other machineries, on reducing design time and design mistake, enhancing consistency and easy documentation， therefore liberating designers from redundant day-to-day routine computer-paperwork to make more innovative contributions to their job. The impact of the design-automation technology on machine design is great.