Abstract: By three dimensional numerical simulation, the fluid flow in the lower plenum was carried out under different annular downcomer thicknesses and coolant velocities. The mass flux distribution of the coolant flowing from the lower plenum to the reactor core was investigated, and the minimum deviation of the coolant mass flow rate through the flow plate was found, which was between these conditions and average coolant mass flow rate. The coolant flow field and temperature field around fuel plates were studied under the minimum mass flow rate deviation condition and the average mass flow rate condition. The results show that the uneven of the coolant flow field in the lower plenum because of the difference of the annular downcomer thickness and coolant velocity can make great influence to the coolant flow field and temperature field in the core.