<p>For controlled nuclear fusion, it is of significance to develop a comprehensive simulation environment for inertial confinement fusion (ICF). This environment must accurately calculate the energy loss of charged particles in high-temperature, high-density plasma and simulate the physical parameters of the fusion reactions and products. This paper presents a novel implementation of a modified Li–Petrasso (MLP) energy-loss theory within the Geant4 framework to address the critical challenge of simulating charged-particle transport in high-temperature, high-density plasma for ICF research. The modified theory integrates binary collision terms, collective plasma effects, and quantum degeneracy corrections, enabling the accurate calculations of the stopping power, mean collision path, and energy transfer dynamics for particles, such as recoil alpha particles, deuterons, and tritons, under extreme plasma conditions. This study introduces how to embed and calculate this process within Geant4 and verifies the correctness of the embedded model. A complete simulation of the fusion process was also conducted. The results demonstrated that the improved Geant4 can effectively handle the energy loss of charged particles in such environments, calculate important fusion parameters, such as the neutron energy spectrum and energy transfer ratios, and observe the production of ultrahigh-energy neutrons. Comparisons with experimental fusion data showed significant improvements in consistency, confirming the validity and accuracy of the improved Geant4. This study has, for the first time, achieved the full simulation of charged-particle energy loss and secondary neutron spectrum of ICF using Geant4, providing valuable insights into ICF characteristics and aiding in the development of more accurate fusion simulations.</p>