This paper is concerned with the practical application of closed-loop control on the gas pressure during the gas-assisted injection molding process. In principle, the gas-injection process can be treated as a valve-controlled tank charging. One of the critical factors for the successful application of the process is the obtaining and maintaining of a desired gas injection pressure within the polymer melt in the mold cavity. Most of the currently available commercial gas-injection machines operate either in an open-loop manner or with closed-leap control, relying upon manual tuning of the associated controllers. This is not only time consuming but also lacking in measurable performance. To improve the controller tuning problem while retaining the currently used controller structure, this study implements a control method which combines system identification and optimal linear quadratic regulator theory into an analytic approach. To demonstrate and verify the proposed method, several experiments were conducted. The results of these experiments show that the approach has excellent potential for designing a control system for regulating gas pressure variation during the gas-assisted injection molding process.