Abstract: The detection of special nuclear materials is related to homeland security, and a fast, safe and reliable radiation detection system is very necessary. A large area neutron gamma detection system based on plastic scintillators was established by using Geant4 Monte Carlo simulation software. The key parameters related to the detector, such as the different cross section side length of single scintillator crystal, multiple layers structure and different natGd₂O₃ wrapping methods, were studied. The detector’s n/γ discriminating performance was evaluated by adjusting the detector’s response number threshold. The results show that the array detector can achieve a neutron detection efficiency of 23% and a n/γ ratio of 802 when a single plastic scintillator with a side length of 6 cm×6 cm is used to wrap natGd₂O₃ with a thickness of 25 μm, a Pb plate with a thickness of 3 mm is placed between the detectors and the detector response threshold is set to 3. The calculated results provide a theoretical basis for the optimization of key parameters of large-area array detector.