Abstract: Three dimensional computational model of the fluid flow in the lower plenum of a pressure water reactor was built up. The flow field in the lower plenum was computed under the condition of different annular downcomer thickness and coolant velocities. The effect of the annular downcomer thickness and coolant velocity on the mass flux distribution of the coolant flowing from the lower plenum to the reactor core was also investigated. If the annular downcomer thickness or the inlet velocity is different, the location, scale and velocity of coolant eddies will vary, and the mass flow rate distribution at the holes in the flow plate will be different. At lower inlet velocity, the mass flow rate of every hole group on the flow plate fluctuates near the average value and the hole group of maximum mass flow rate locates at the center of the flow plate. While the mass flow rate decreases from the center to the edge of the flow plate at higher inlet velocity. The hole group of maximum mass flow rate moves from the center to the edge of the flow plate with the thickness of the annular downcomer increasing.