Abstract: Geant4 software can not only simulate the interaction processes of rays and material but also the following transportation of optical photons produced by these processes. In this study, the Geant4 particle transport simulation platform was established. The optical coupling efficiency calculation model of the nuclear fusion -ray energy-time joint spectrum detection system was constructed. The optimization method employing the genetic algorithm to call Geant4 code was proposed. The parameters of optical coupling structure were optimized by the genetic algorithm using the gap length in the front of the grid lens, the length of the grin lens, and the gap length at the rear of the grin lens as the independent variables and using the coupling efficiency as the fitness function. Optimization results show that the coupling efficiency between the scintillator and optical fiber could be improved with this method, which is about 4.77%.