Abstract: An on-line prompt gamma neutron activation analysis (PGNAA) system was designed by using Monte-Carlo simulation for the thickness of different moderator materials （heavy water, paraffin wax, polyethylene, etc.）, the thickness of shield lead, and the thickness and size of sample. The calculated results indicate that the optimum dimensions are Φ50 cm×9 cm for paraffin as moderator material, Φ20 cm×2 cm for lead as gamma radiation shield, and Φ20 cm×7 cm for S, Ca, and cement samples. These parameters provide references for practical design of the on-line PGNAA system.