Abstract: Measurement of fusion reaction history by detecting the 16.7 MeV gamma-rays needs high neutron yield. For this reason, simulation study for fusion reaction history diagnosis with inelastic gammas generated by 14 MeV neutron interacting with the puck in front of (Gas Cherenkov Detector, GCD) was carried out. Material and thickness of the puck were determined by calculating energy spectrum of the secondary gamma and yield of gammas above the Cherenkov threshold. Cherenkov signal of the inelastic gammas through the GCD generated by interaction of 14 MeV neutron with the puck was simulated. Noises of gammas, electrons and positrons at the photo-electric conversion device were analyzed. Relation of the statistical fluctuations of the Cherenkov detection system and the fusion neutron yield was ascertained. Fusion neutron yield for fusion reaction history diagnosis by detecting 14 MeV neutron can decrease by two magnitudes in comparison with that by directly detecting 16.7 MeV gamma rays.