Abstract: Plastic scintillator detectors are widely utilized in radiation measurement due to their unique characteristics such as fast decay time, low production cost, availability for large-scale production, and tissue equivalence. However, their limited photoelectric absorption cross-section and poor energy resolution make it challenging to accurately detect γ-ray photopeaks. The absence of high-energy γ-ray photopeaks poses a challenge in the energy calibration of plastic scintillator detectors, which is the foundation for radiation energy detection. Nevertheless, it is feasible to calibrate a plastic scintillator detector using the Compton peaks. Numerous studies on the plastic scintillator detector energy calibration based on the Monte Carlo method were reported, but most of them cannot directly simulate energy spectra or calculate the energy values of Compton peaks. An energy calibration method combined with optical simulation was proposed to overcome this obstacle in this paper. This method describes physical processes in plastic scintillator detectors more comprehensively than energy calibration methods based on the regular Monte Carlo simulation. Furthermore, it does not rely on empirical formulas to determine energy resolution. An optical model of the plastic scintillator detector built with GEANT4 10.7 was used to describe the scintillation properties of the plastic scintillator, the properties of optical surfaces, and the photoelectric conversion and photoelectron multiplication processes of the photomultiplier tube. Thus, this model could accurately simulate the energy deposition of γ-rays, the transportation and collection of the scintillation light in the scintillator, as well as the photoelectric conversion process of the scintillation light and photoelectron multiplication process at the photomultiplier tube. These allow for an accurate simulation of the experimental energy spectrum. The simulated energy spectra were calibrated using the photopeaks of low-energy γ-rays. Since the simulated energy spectra are reasonable approximations of experimental energy spectra, the energy values of the Compton peaks in the experimental spectra are the same as Compton peaks in the calibrated simulated energy spectra. Two radiation sources of ¹³⁷Cs and ⁶⁰Co were used to calibrate the plastic scintillator detector in the experiment based on this method, and the energy calibration of the plastic scintillator detector was completed using Compton peaks of ¹³⁷Cs and ⁶⁰Co. The relative error of the Compton peak energy resolution for ¹³⁷Cs between the simulated energy spectrum and the calibrated experimental energy spectrum is 0.70%. For ⁶⁰Co, the value is 0.91%. Either value is less than 1%, which in turn verifies the agreement between simulated energy spectra and experimental energy spectra, and also demonstrates the reliability of the energy calibration result.