Plastic scintillators have been utilized for radiation detection in the fields of security screening and dosimetry because of their unique advantages such as low price and tissue equivalence. For research on radiation measurements, it is important to simulate radiation detectors that are as close to a real system as possible because of national regulations on the use of radiation sources. A Monte Carlo simulation with a Gaussian energy broadening (GEB) function is effective for simulating the measured spectra of plastic scintillators. However, it is difficult to determine the input for GEBs and to conduct energy calibration because of their poor energy resolution and the absence of clear full energy peaks in the measured spectra of plastic scintillators. In this paper, a method is proposed where the parameters for the energy calibration and the gamma response function are calculated simultaneously using an optimization technique. The performance of the proposed method was demonstrated in a case study of an NaI(Tl) scintillator. Then, application of the proposed method to a polystyrene detector showed satisfactory agreement with measured spectra.