Abstract: Using the plastic scintillator and polymer matrix composites material as the detecting medium, a new type detector was designed to discriminate neutron and gamma in pulsed n/γ mixture field. By measuring the relationship between the fluorescence and Cherenkov lights, a detection system consisting of a plastic scintillator detector and a Cherenkov detector was used to realize the neutron and gamma discrimination. Then, the pulsed neutron flux can be obtained through the difference between the counts in scintillator and Cherenkov radiator. Monte-Carlo method was used to optimize thickness of plastic scintillator, and give the spectra of charged particles produced by the detector interacting with the neutron and gamma ray. At last, the refractive index of the polymer matrix composites was calculated to ensure that only the secondary particles of gamma ray can generate Cherenkov light.