Abstract: Based on large eddy simulation (LES) turbulence model, the transient flowing field in the upper plenum of PWR was simulated to obtain the coolant temperature fluctuating. The results show that temperature fluctuating phenomenon can be obtained effectively by the LES model, and the large temperature fluctuation is more concentrated in the low frequency part without apparent period at the exit of the upper plenum. The temperature distribution is more influenced by the exit position, from the entrance to the middle plane corresponding to the center line of upper plenum exit, and the temperature fluctuating amplitude is larger at the peripheral and central section than at other regions. The whole temperature fluctuating slows down gradually from the middle plane to the top.