Abstract: The multi-layer annular fuel assembly, which is consisted of annular-type fuel plates, is easily blocked. It is necessary to study the flow and heat transfer behavior of the assembly under flow blockage condition. In this study, basic problems in three-dimensional investigation on flow blockage accident in multi-layer annular channels are discussed. Scalable wall functions are proved to be suitable to simulate flow-rate-decaying cases. A 90% inlet flow blockage case is then calculated using velocity-inlet and pressure-inlet respectively, and the pressure-inlet is proved to be the proper boundary condition for CFD simulations on flow blockages. Both transient and steady simulations are done, and flow and heat transfer redistributions caused by the blockage are analyzed.