[en] In-flight calibration is crucial to monitor possible variations of detector performances, especially the polarimetric response, in Polarimetry Focusing Array (PFA) onboard the enhanced X-ray Timing and Polarimetry (eXTP) observatory. A physics-based model is established in this work to perform a quantitative simulation of the in-flight calibration sources, utilizing the GEANT4 toolkit. The physics of Bragg diffraction is implemented, derived from the scattering theory, and validated by the experimental measurements in terms of the diffraction efficiency and the degree of polarization. The extended physics process provides spectral characteristics and polarization states of emitted X-rays with reasonable accuracy. It is expected to play an important role in advancing further development of the instrument. Additionally, it enables the integration of multiple instrument simulations within the already existing framework of GEANT4, which facilitates the study of the space mission.