Abstract: The swimming pool reactor with planar fuel assembly was studied with the computational fluid dynamics program CFX5, and the flow field in reactor was numerically simulated. The results show that the velocity in fuel assembly is high, and the velocity reaches its maximum value at the outlet of the fuel assembly. Whether or not the reactor is installed with enclosure, the main pressure drop is focused in the fuel region. The pressure drop in the fuel region increases with the increase of the entry flowrate. The pressure at the upper and lower cavities in pressure vessel varies little. The differential pressure between inlet and outlet of the reactor core with enclosure is closed to the differential pressure without enclosure at the same entry flowrate.