Abstract: To clarify the effect of counter-pressure on the dimensional stability of thick-walled plastic products, a series of experiments were performed for injection-molded thick-walled plastic products by using self-developed solutions. The influence of backpressure on the shrinkage of thick-walled plastic products was also observed and analyzed. Furthermore, a counter-pressure injection molding method is proposed. In this injection mold, a piston was incorporated into the cavity of the mold, and the pressure element was connected to the pressure element to provide backpressure. The injection pressure of the melt was used to push the piston to move, thus stabilizing the melt in the mold cavity. The pressure was maintained in real time, thereby reducing the internal stress of thick-walled plastic products and obtaining an ideal product with a small shrinkage rate. The experimental results indicated that the use of acrylonitrile-styrene copolymer as a raw material for counter-pressure injection molding of thick-walled plastic products could effectively reduce the shrinkage of thick-walled plastic products, thus achieving a better dimensional stability for thick-walled plastic products. Meanwhile, the holding time for pressure could also be significantly shortened, leading to an improvement in production efficiency.