Parameters of fusion reaction history play an important role in inertial confinement fusion diagnosis. Two types of detectors, named gas Cherenkov detector (GCD) and gamma reaction history (GRH), have been well applied for measurement of fusion reaction history due to their fast responses and capacities for setting the threshold. This study was carried out in two stages. First, simulation of some components of the GRH system was carried out with Geant4. Second, an optimization method by combining a genetic algorithm with the Geant4 code was established and applied to the optical reflectors of the GRH system. The optimization process was focused on 16.7-MeV gamma rays with a threshold of 12 MeV. An optimal time response of 5 ps and an efficiency at the receiving surface of 2.2661×10−2 Cherenkov photons/incident 16.7-MeV gamma ray were obtained at 1.9158 atm of CO2 pressure and a temperature of 20°C.