Abstract: In the Pebble Bed Fluoride-salt-cooled High-temperature Reactor(PB-FHR), the pebbles are loaded by buoyancy in the coolant. Hence, the packing structures of the pebbles in the core are greatly influenced by various factors such as the core structure, the loading rate, the coolant density, and the coolant flow. The uncertainty of the pebble bed structures is the key issue in the reactor design and safety analysis. To provide a mock up for the Solid Fueled Thorium Molten Salt Reactor (TMSR-SF1), the Pebble Recirculation Experiment Device(PRED) was developed. Packing experiments has been performed under different conditions. For the purpose of verifying the design, the study on pebble structures has been performed based on the TMSR-SF1 design using Monte-Carlo code MCNP. The results quantitatively evaluate the effect of various packing fractions, bottom shapes of the pebble bed and the loss of coolant on the key neutron physics parameters. It shows that, when the packing fraction is increased from 50% to 64%, the effective multiplication factor, the pebbles loaded and the temperature coefficients are increased, but the control rod worth is decreased; the bottom shape of the pebble bed has different effects depending on the structures. The positive cone can cause both increasing and decreasing of the reactivity, and the inclined plane and the negative cone cause a decreasing of the reactivity; the loss of coolant causes a denser packing which results in a decreasing of the reactivity.