The Rapid Prototyping (R.P.) technology which is used in manufacturing a three-dimensional solid part is based on the layer forming method. Currently, the main streams of commercially available machines use the point or line forming method to create the solid part. A thin polymeric material layer is delivered by a roller or scraper to form a thin material layer for further layer forming. This process won't be able to control the size of the forming part. In this paper, "Liquid Spray Process" is proposed to use a spray pen to spread a thin layer of polymeric material on the forming bed for further layer forming in a rapid prototyping system. It will decrease time and cost of the current R.P. machines by spreading proper size of thin forming bed. The setup of the system of the automatic spray pen, continuous material supply system, spray parameter settings and man-machine interface control system were discussed in this paper. Furthermore, the Taguchi Methods were applied to find the optimal spray forming parameters including height, velocity, acceleration, control air pressure, atomizing air pressure, fluid flow of spray, material pressure and speed of gear pump. Finally, the minimum uniform coating thickness could reach as low as 25 μm with "Liquid Spray Process" when the optimal parameters were used. The uniformity could be well maintained for both sprays in cross or in parallel directions to form an area of 120 mm × 150 mm with the optimal parameters.