Abstract: An optimization method combining Geant4 code with genetic algorithm was established and applied to designs of two types of gas Cherenkov detectors (GCDs) because of deficiencies of the traditional optical ray-tracing method. Improved efficiencies and time responses were obtained compared with results of traditional optical ray-tracing method. The efficiency of GCD system through optimization is enhanced by 20% while time response is eliminated by 7.2%. The efficiency of GRH (gamma reaction history) system through optimization is enhanced by 91.2%. Aiming at fusion diagnostic process, the MRNSD (modified residual norm steepest descent) method was ascertained as signal recovery method for fusion reaction history diagnosis through comparison study. The MRNSD method can recover sub-nanosecond signal distorted by response function of which the FWHM (full-width at half maximum) is 10 times of the signal to be recovered and with 10% Gaussian white noise added, which is suitable for fusion reaction history diagnosis.