Abstract: he optimum neutron activation analysis system is designed by using Monte-Carlo simulation for 6 kinds of moderator materials, and the optimum moderator material is polyethylene. The thermal neutron flux density distribution of the shield equipment adopting polyethylene as the moderator material was experimentally measured, and the optimum position of the neutron source and the detector was determined. The results lay the foundation for the study of on-line neutron activation analysis system.